Igor Well there are two methods of doing this. I've used the one that doesn't break the game's terms of service. I did the following: Go to the ingame store (that is optional, screenshotting the locker could also work considering you just need only the character model captured for this one.)The higher the resolution and fidelity the better, this one was initially 11k, res and color settings if needed: 
but upon upload/embed it got extremely compressed and downscaled. Pick any character you like, capture a high res photo of them, get rid of the game's interface in photoshop or illustrator i do recommend illustrator due to how it handles image quality. Do some masking, shading, color correction and touch ups like filters and manual corrections to preserve the character's true colors which is also optional i added shading and contrast to the character on this one, blend in with the background (small amounts of motion or gaussian blur applied to the character's layer do wonders for blending) and that's pretty much it. Honestly nowhere near to being as good looking as the original wallpapers provided by the game's company but i do believe those were straight up rendered in unity so they have the better raw lossless quality and lightning.
The second method i am unable to disclose due to it being against both the forum and game's TOS but it will work way better then screenshotting ingame characters.
