If it is an existing mod, give a link to it (do not upload it again).
Note that you have to ask the permission of the original modder for a revisioon of his mod (send him a PM, if he doesn't answer in two weeks, it's considered that there's no problem).
If it is a new mod, you have to provide side view, anime-type references. (if you're asking for something to "look like Cream the Rabbit's fur pattern", you have to provide a reference of this pattern as well). You have to host your references on an offsite image hoster (e.g. Imgur).