There has been some kind of Flickr integration in iPhoto for a year and a half or so, although I’ve never had that version, so I don’t know how good it is.
Or, you can drag the photo out of iPhoto itself onto your desktop (or wherever). This will make a copy of the photo which you can then upload some other way. Or you can Export the photo in whatever format and size you want, to a new file, and upload that.
Commenting is turned off on this blog.