Scenario:
i download a picture of a rendered (png) character in fortnite, from a website and with this picture i create a youtube thumbnail.
Say I have my own background and pack on it this image (rendered charackter) and save the whole then as a new file and then upload it to youtube, as thumbnail z.b.
now my QUESTION: can it be proven technically that I have downloaded the image from a website and have not made the effort myself to cut out the character (render)?
Unfortunately, I can't answer exactly how this is in your case. But: with normal photos, it is no problem for me to identify myself as the author, since I'm the only person who has the original photo in RAW format. Anyone else can "only" have a JPEG version of it.
Each copy also has an original. If a copy can be assigned to the original, then the copyright infringement is proven. Special software filters for such violations.
Also: You can hide your copyright in pictures. All you have to do is leave your logo in a monochrome area with a slightly different color. The layman does not find this logo, the author already.
Let's say, somebody will have drawn the figure yes, if the recognizes and insists on his coppyright, it could be hard.
however, if you're not called sony bmg or warnerbrothers, it's quite difficult to enforce your copyright. That That many small, simple people refuse to get their hands on it.
but the chance still exists. Therefore, I advise you to draw something of your own, render it yourself, and then, aside from the fact that others may do it just as you did before, you are on the safe side.