That's not how iPhoto is intended to be used. They don't want you to browse the actual files for your photos, it's a mess in there.
When you want to mail photos you're best (IMO) to open iPhoto, drag the pics you want out to the desktop (this will makes copies, not remove them from your library). You can then edit, rename, whatever the copies there and mail them off.
You might even be able to mail straight from iPhoto (I forget, and am not in front of my Mac right now).