Hi there, and sorry for tackling your question so late!
We deliberately wanted to leave the contest open to creative submissions that are not only paintings or drawings, so collages or manipulated screenshots definitely count as long as there is a reasonable amount of originality and authorship in them: Selecting and manipulating the snippets, making it go together as an unique idea etc.
As long as it doesn't become a fully derivative art of two photos pasted together, we will definitely consider it for the contest.
I understand it is a thin line, but if you look at submissions like for example @Raq's Vile Bile in the bathroom, that for me would be an original artwork created using stock footage. The creativity shines through.
Concerning the theme, we will certainly for this contest award a submission that depicts one of our heroes in a positive way. Still, there are some great versions of them shown off here that I wouldn't right away dismiss as fan created because they are clearly taking our heroes as the base, for example the cool looking Firemage from @GibMirOpfer !
When it comes to "Being heroic!" - that is left very open as to how you want to depict them. For this I think @Warrior_Gracchus_V made a great submission with the pet saving awesome Megataur being heroic! But it doesn't need to be on-the-nose, and is open to interpretation: Striking an epic pose or showing off, mid-battle, of even goofing around.