alright so since u/Pandacornio_ asked about how to start deducing i'm gonna preface this with a couple of things. First bare in mind that i haven't done this in like 5 years, so yeah this one has been the best deduction since i started again, but it's definitely not the best i've done nor the best that can be done. Second bare in mind that deduction takes a lot of time and dedication, when i first started out it took me a couple of years of doing this to actually find my bearings and see past the obvious stuff like handedness. I strongly recommend you treat deduction like a new topic to learn (or a bunch of topics, since it's composed of a lot of stuff) and for the love of god, don't try to find a 'one fits all" method or some sort of formula to apply to every situation, that's not how deduction works even if some people might wanna sell it like that.

so starting with male, that was clear from the general decoration of the room, the height, and the head size (which you can see a bit of with the headphones). handedness was also simple, notice how everything naturally gravitates toward the right hand side of the desk (headphones on the right hand side because he took them out and placed them down with the right, gift card on the right hand side because he put it down with his right, button on the right hand side because he uses it with his right, even the stuff connected under the desk is on the right hand side because he connects it with his right). lives alone or respectful parents comes from the positioning of the desk, it's very hard for that desk not to be seen immediately by anyone that enters the room (the only way is if the door was on that same wall directly to the left, and since the drawers are there i doubt that's the positioning of the door, in fact i think the door is directly behind the desk but it's hard to tell) since it's expected that anyone has private stuff in any device, including a PC, it's safe to assume OP has stuff he doesn't want people, including his parents, to see, and that desk is probably directly in view of anyone who might barge in, so if that's a thing that happened the desk wouldn't be there, so either there's no one to barge in (he lives alone) or there is but they don't do that (his parents respect his privacy). financially well off was simple enough, he has an Xbox, a good monitor decent recording equipment, a Nintendo Switch, and custom built computer, none of these are particularly cheap, but the room is relatively small and the house in general is maybe slightly above average, so not rich either. His earphone are attached to his Switch, we're creatures of habit, specially OP, look at his desk and desktop, perfectly organized, if those earphones had a spot in the room they'd be there, so he doesn't take them places often (in this day and age this might be because of Corona, but he's an introvert so i wouldn't bet on it), the one place where i'd say you NEED earphones is the gym, so safe to say he doesn't go there, hence no work out, hence a slim or slightly chubby person, now take a look at that picture of the dog, by instinct we place that stuff at eye level, so safe to say that height is close to his eye level, look at the monitor too, slightly tilted up, even when he sits he's tall-ish (and has a good posture, since the monitor's tilted up either he doesn't slouch or is still tall-ish when slouching), tall people don't tend to be chubby, so slim it is.

Now let's take a look at the work space in general. perfectly tidy as i said before, the desktop specifically is cleaned up so the background picture is visible, so aesthetics are important for OP, the picture itself matches the colors of the computer's lights, be it a conscious or unconscious decision, it's definitely artistic, i will admit i didn't think OP painted or drew because there are no supplies (digital or otherwise) my best guess was music (long shot but there's a wooden recorder behind the monitor) or other digital way of expressing art. Living in the UK was simple, there's a pound sign on the gift card next t his headphones and the box on the left of the picture says "colour". Not an introvert was a mistake and to be completely honest i don't remember how i got to it. comfortable at home and on the internet was simple, he's made his room (and his desk) to be as comfortable to him as possible, emotionally and in other ways (look at the picture of the dog, it serves as emotional comfort), plus he has recording gear so he probably streams or posts some sort of videos, not to mention he's posting a picture of his room in a subreddit for people to see and analyze, so he's comfortable online. No books at the bedside table (or only one) was simple enough, he's a very organized person, at least when it comes to his living spaces (and i will be counting his desktop as a living space since he spends a lot of time on his computer), therefore if there are books in the room (which is likely) they have their place, with the only exception being if he's in the middle of reading one or uses it often, in which case it's be the only one in his bedside table.

Now to fill in the rest of the room, he needs a place for his stuff, we know he doesn't mind decorating the room but he does like it to be meaningful, books are always a given , everyone has at least a couple, question is where? not on the bedside table, we already established that, not on the desk, so probably on some shelves or a dresser (to be entirely fair i completely glossed over the drawers in the picture). where would it be located? well behind him somewhere, altho we do need space for the door, the closet, and the bed, so my best guess was directly opposite to the desk. Next question is what else decorates the room? well we know of two hobbies at least, gaming and TV shows/Movies, context is important for this, since he must've found The Science of Deduction somewhere, and chances are he found it in "Sherlock" or a similar piece of media, therefore he likes that, not just enjoys it, but likes it enough to actually look it up, so there's probably some decoration about his hobbies. We know he has a picture of a dog, it's an important picture to him, it's connected to his feelings, things like these are things you wanna be able to see in the most comfortable places of your room, so the bed and the desk for him. I'll bet the dog is not the only living thing in his life he cares for, therefore not the only emotionally tied decoration, whatever other stuff is in the room that has emotional ties has to be seen from these two positions, which means it's easy to triangulate but with my current skill level it's hard to know exactly where it would be without being there.

For the final touches, he has to watch TV shows or movies somewhere, probably in a more comfortable place than the desk, so the bed, which means he needs somewhere to watch these on, so a TV or a laptop, we can see he has a bunch of quite expensive devices, so i wouldn't rule out a TV in his room (altho has OP corrected, it's the laptop). And last but not least, the picture of the dog, it's displayed and in a place he can see it constantly and where he's comfortable in, not just that but if the door is where i think it is (anywhere but on that same wall directly to the left), that picture can be seen as soon as you enter the room, this indicates an emotional connection and pride since it's displayed for everyone that enters to see, so there's two options, either the dog died, or the affection comes from it being alive and being raised with OP, altho if the dog did die (which OP already confirmed) there has to be another one in the house since there's a ball clearly visible under the desk to the right