Unfortunately, there are no official regulations that address these issues.
I must admit that I myself am wondering how to deal with this problem in the future and have no idea. In the context of AI-generated images, it could be considered that the solution could be to show the stages of the work (screenshots from a graphics program showing sketching, line art, coloring, etc.), to bet on work done manually, etc. But, in my opinion, this does not solve the problem either, as it is trivial to simply redraw the graphics by taking the generated image as a base (model). I am also assessing this problem on my own abilities as a contestant. Hypothetical situation: I generate a graphic, create a sketch based on it from scratch, and present it in the steps of creating a drawing. Is it possible to prove such cheating? No.
The only thing I can think of is to choose artwork that depicts drawings of Tibian characters/creatures so that they look like they are from Tibia, rather than referencing common images from the Internet.
A good example of this was the recent TibiaDraptor.com competition, where there were about a hundred entries posted, some of which were generated and disqualified: they all had a common denominator - the draptor more closely resembled a raptor from Jurassic Park, rather than a creature known from Tibia.