As long as they're your original creations, if you can introduce characters and work a story with them in the contest limit (2500 words), feel free to submit it. Just don't rely on the comic book to explain the story or anything like that.
EDIT: My reasoning for this, in case anyone is going to ask, is that the regulations of the contest stipulate that "you should write something new within the time frame of the submission stage" (emphasis added). This has no bearing on whether or not your ideas are fresh during that time frame. That would, after all, be a ridiculous rule, since it's fairly difficult to a) come up with any completely original idea, no matter how long you're given, and do so on command within just a few short weeks (and then write a polished story with the idea). So, since it's quite a challenge to convert a full-fledged comic book (the idea) into a independent short story (writing the idea), given the time and word constraints we have here, I would allow it.