Roland pointed out iPhotoToWeblog, which is an AppleScript for posting to metaWeblog-API compatible blogs.
I have always used Gallery for my mass posting of pictures, because I really do have too many pictures to clutter up a blog with – like around 8,000 from the past 2 years or so.
Since I’m on a Mac, I use iPhotoToGallery, which is completely integrated into iPhoto with an “export” plugin that lets you post directly to any gallery without leaving iPhoto. It even supports great Mac integration like saving your password in the KeyChain and resizing pictures locally. I had talked to Zach a while back about adding MetaWeblog API support. He’s apparently toiling away at it right now. Go make his traffic spike to convince him to release what he’s been working on!