I feel like it would really help revitalize this place, if there was a stable place for people to go to contribute free works for the betterment of the community.
The fact that all of the sites you mentioned are either crippled or gone seems more indicative that the "return" they provided simply did not outweigh the cost.
There are actually more free and convenient file sharing sites than ever before (with the BIG ones being sites like Dropbox, Google docs, Onedrive, etc.) Not to mention all the photo sharing sites and places like Pinterest. If people really wanted to take the time to share something it has never been easier. Finding what you're looking for, however, does require a little more effort due to the sheer number of places to search.
Honestly, just doing image searches on Bing or Google often brings up repositories of specific assets you might be looking for.