When using an external editor such as Photoshop Elements, typically you should be saving your edited files outside of the iPhoto Library, say on Desktop, for example. The iPhoto application does not read any library changes that aren't administered by itself. To retrieve the edited images, you'll have to access the contents of the iPhoto Library via Finder. Inside ~/Pictures is an iPhoto library, which you have to secondary click in order to get the Show Package Contents option. Normally iPhoto-edited images would be in the Modified folder, but saving them through Photoshop probably wrote them into the Originals folder. Be extremely careful digging through here because renaming/moving any of the files in here could require rebuilding your entire library database.