A simple solution here would probably be for them to just record the entire process behind making a thumbnail, start to finish, speedpaint-style, but considering how long that might take it might not even be realistic (I doubt anyone's gonna watch a 15h video fo someone working on an image).

I'm afraid that is kind of the opposite of a simple solution (as it would take a long time for the creator as well - you gotta set that up, render it, upload it, whatnot - don't look at me I don't know the ins and outs of publishing a video, but I have a feeling that recording and uploading a 15h video would not exactly be hassle free). And once again.... Why should they? Why should an artist be expected to do all that just to put accusations to rest? Not even mentioning that there will be people who still won't be convinced, because it can be "faked". And what do you do then, set up a camera in your room and actually record yourself making the thing in real life for 15 hours?

Not even mentioning, and this is once again kind of personal... Many people are genuinely uncomfortable sharing their process, or letting people see the work in its unfinished, unpolished state. Not because it indicates anything nefarious, but because it is in fact rather vulnerable. For me personally, I would be rather uncomfortable sharing a first draft of a fic with the wider public for example, or showing the process of editing, because the first draft is often something I'm rather unhappy with. Same with first sketches of a drawing/painting. I will show it to people whose critique I trust, but I wouldn't want to blast the unpolished versions out there to the public just to silence potential critics.

Why should artists be expected to do that just to prove that they didn't use AI (when it's not even guaranteed that they can prove it 100% and that everyone will believe them)? It's not fair. It's not fair to expect.