Protip: You get what you pay for, and if you are not willing to pay a single dollar for the artwork of what you purport to be a serious, organised convention, then you deserve some half-arsed deviantart crap full of comic sans and fail. This is rather like walking in to a garage, and expecting a mechanic to fix your car, providing all his own parts and labour, simply for the satisfaction of a job well done. This is not how the world works. A professional graphics designer or illustrator can spend anything up to five years in higher education, and proffessional quality art materials, software etc. is extremely expensive. To waltz on to a forum on which you have not even lurked and asking outright for about $300 worth of work (at least) done free demonstrates almost unbelievable hubris. Perhaps amongst fans of the convention, or your own personal friends, you might be able to swing it, but dude, seriously.
If you really are so fucking stingy that you won't pay out for quiality product, I suggest you either
a) go to deviantart with this shit. You'll get everything you need, though I would be incredibly surprised if it was all in the correct formats and resolutions for the purposes you require, and even more surprised if it didn't blow chunks.
b) start a competition on the forums, myspace, whatever for the convention itself. Offer some cheap-ass prize like t-shirts, or pocky, or the opportunity to feverishly lick the genitals of whatever third rate english-language Seiyuu's you may have managed to pursaude to attend.
Further note:
Signing yourself off in every post when the forum automatically arrays a signature block beneath each of your posts and displays your username is sheer dickery. Stop it at once or I shall be forced to attend your convention wearing only a three sizes too small Sailor Saturn outift and open fire into the crowd with a sawn off shotgun.