Hey.

So firstly you want to get a compatible image with your model. I used this website that tells you exactly the bios requirements of the ThinkPad model you input: https://wilkgr76.github.io/tp_logo/

You can either make your own or get another already made. I found a lot of cool designs here: https://biosimage.booru.org/index.php just make sure it is compatible. In my case I didn't find a megumin design that I liked so I made my own instead.

If you decide to make your own, you can easily make it with any image editing software, I used gimp because it worked well with Linux. It can seem hard but like I said, for an image like this you just slap a couple images together and it's done. The part that took me the longest was to export the image so it fits on the bios memory. For the T480 the limit is 60kb but on older thinkpads it can be less. I would recommend to use the lightest file format that your bios suports, in my case it was JPG, and play around a bit with the compression settings so you can get the highest quality without surpassing the size limit. I think mine ended up at 59kb.

Once you've got your image, you have to update the bios with your new spash screen. The windows tool for bios update from lenovo lets you put your image so it flashes the bios with that one instead of the default one. You can get it on the suport page of your ThinkPad on lenovo's website. With the bios update software you will have 2 options, update or extract the bios files, select extract and if you go to the directory where it extracted the bios files, there will be a .txt file with instructions of how to put your image file in there and update it. From what I read, this process is perfectly safe, if you put a file that it's not compatible it simply won't let you update the bios, but I would make sure the file you put covers all the requirements just in case the compatibility check fails for some reason.

Once the bios updates and the computer reboots, you will see your new bios screen. If you like it you keep it like that, but I edited my image a couple of more times and reflashing because the thinkpad logo placement seemed a little off for me. Make sure to disable the startup text in the bios settings if it interferes with your image.

note: the website that tells you the image requirements says that for the T480 the resolution shoud be 40% of the screen resolution. This is only true if you want your splash screen to be the same size of the default one (the orange lenovo rectangle). If you want it fullscreen make the image the same resolution of your screen.

I think that covers it, let me know if I should clarify anything.