Would this not be Fan Art?You could devide the two 'terms' up this way:Fan ArtArt/graphics created (from scratch) based on a topic/theme/characterPermission required art/graphicsArt/graphics created/based on/modified from already existing art/graphics
That's a bit of a misconception also.
Fan art can be someone using an image that was created by someone, like taking an image of superman and using it as your desktop wallpaper. Technically it's fan art but you didn't get permission from DC to use it as wallpaper and I'm sure DC didn't give permission for that art to be distributed on the net and to be downloaded by other people to be used for that purpose.
Fan art can also be someone creating a replica of a popular image, example: drawing a pic of superman. You might think this is harmless but in the end it still represents copyright infringement. Even if you drew the pic of superman (example only) on a napkin for your own personal use, it's considered a derivative work and goes against the copyright that DC owns on the superman image (or whoever owns it).
There is a catch with fan art though, this is where it gets grey. If you don't use the fan art to make money and you're not trying to make a profit of this fan art (example, drawing superman in photoshop, creating a wicked wallpaper and allowing other people to download it from you & use it as well), it can be considered fair use (somewhat).
The mods on this site make the final call on what they will allow so submit what you want and they will approve or reject it accordingly.