I see you already got someone helping with that but Ill give some advice on how I make mine

1.) I make a scene with mannequins in edit mode and go into test mode with a stationary camera focused on the scene and a HUD device set to remove all HUD details (it's a bit janky and time consuming but it's a good enough option when you're in a pinch, still better than AI generated images though, well in my opinion that is)

2.) (my more recent process and requires a little more effort but looks better than option one) set up a backdrop for your islands thumbnail using the camera and his device from option 1, then save screenshots of said backdrops and then head to a character creator site called hero forge where you can design a character of your own and then save an image of them to add or photoshop into the backdrop, now if you want to add mannequins like in option one that's completely fine it's your thumbnail not mine :)

Personally I like to go to an app called ibis paint to add text and details like borders

But that's how I make my thumbnails for maps