try to search in 'graphics' or 'postershop' tag
http://www.asianfanfics.com/browse/tag/postershop

then fill the request form they had, the shop also might have some additional rules.
or else you can request through the "request" option, on the page list of your authored stories there's "request" button, click on it and fill the form, don't forget to tick on the graphics option and publish it.

Just look up graphics on here and they'll tell you how?