I have the same problem. I have almost 100 gigs of photos which are the majority of my backup needs.
Flickr's been an somewhat easy way to back them up and share them at the same time. But I have growing frustration with Flickr because I'm really using it for three different purposes now: backing up all my photos, sharing family snapshots with friends and family, and sharing more artsy photography with mostly strangers (which is really Flickr's sweet spot).
Using Flickr for backup is incredibly annoying and iffy because their uploaders are so fragile and downloading the entire collection depends on third-party software that Flickr could break or disable at any moment.
I'd love to move my photo backups off of Flickr and am close to using just some rotating USB hard drives, but I'd much rather have something automated. Dropbox could definitely have my business there and be price competitive if they offered say 250 gigs for under $100 a year. Backing up my photos should be a good profit for them over time, because once I get past the initial backup, I really only upload a gig or two a month and rarely download. Unlike my other documents, I don't need my photo backup replicated between my different computers running Dropbox.
Sharing photos with friends and family is something that Dropbox could be good at or at least acceptable. The Flickr, Picasa, and Facebook uploaders all suck compared to the ease of Dropbox. If they added some better privacy settings, RSS feeds, and some more viewing/downloading options, they could be player in the photo sharing space.