looking for that info too I think social networks might be good as well as forums, not sure what kind though
Myspace is too crowded to promote an image hosting site... Plus it has its own image hosting system. Go to forums and blogs and look for people asking for a good image host. Don't forget to link your site in your signature.
One method that worked for me before I sold my image hosting site, was to use my own watermarked images in eBay auctions. This brought a lot of traffic and sign-ups from others browsing my auctions.
Stumblers have the worst conversion rate on the web. A quick 50 visits from StumbleUpon can easily convert 0 visitors to customers. They tend to only stay on your site for an average of 2 seconds. The only way I have actually found "limited" success with SU, is by directly greeting the SU visitor. Of course this could be an eyesore for other visitors so plan on dedicating a landing page just for SU visitors. Facebook has a newer free service for businesses and websites called a "Fan Page". This works okay if you are active on Facebook. MySpace is getting harder to promote anything other than Music because of all the spam that has overshadowed genuine businesses. If you want to know what really works, PM me. I use a service that is one of the best online marketing methods, but won't post it publicly for fear of it being overused by spammers also. Just ask, and ye shall receive. lol