Have to agree with Martijn. While the tag idea is definitely something that should be done, a grouping interface is also useful. It's pretty much expected and familiar to users. Also not every user comes into a spriting database for a specific thing to search. Like for instance, sometimes I just browse to browse. While I like the idea you have Niek, it might prove an exhausting list when you consider games outside the realm of Zelda. I would suggest a general type though, Tileset, Background, NPC, Player, Boss, Enemy, Custom, Edit, Misc. etc.. etc.. and force a resource into that general category.
On the other hand. The submission system is designed to be automated. Leaving most, if not all the information about the resource on the side of the submitter. Where as on TSR it seems they have the staff to handle things like custom buttons, validation, and proper organization of a submitted resource. So we have to consider these facts, and how far we can take the grouping aspect of resources.
While TSR is great, a lot of sprites aren't really fan game ready as they are. (As in non-consistent frame sizes, item sprites separated from main sprites, and point of origin) I think that can really be something we could handle differently here. Since we are a fan game community, we are in a better position to make quality sprite sheets that are also usable right out of the box. Maybe zfgc approved sprites that follow a specific format?
Also for giggles: