Is it even possible to chose a photo out of your photo librry from a browser's file upload dialog? I don't think it is, when the photos are in iPhoto. How many steps does it take to get a picture off your camera, into iPhoto, then exported, and finally uploaded to Craigslist?
Photos should show up as an icon in the side-bar? Rather than navigating to the actual files I think you're meant to access them this way. It's a good idea and used to work better in my memory - these days it can be a bit finnicky to find the photo you want.