Sure!
1. Open iPhoto.
2. Select the photos you want to upload.
3. If they are going to Facebook or Flickr or Twitter, just use the built-in Sharing menu.
4. Otherwise, drag a copy of the photos out to the desktop, upload to wherever, then trash the desktop copies.
(many other photo services have plug-ins for iPhoto that will let you upload directly, look into that possibility also).
As for the reason why you can't see where the photos are in iPhoto, that's because iPhoto uses a database to work with your pictures, and if you move pictures you corrupt the database, which puts you at risk to lose ALL your pictures. So Apple hides the photo database to prevent users who don't really know what they are doing from messing up and losing pictures.
Bottom line: ANYTHING you might want to do to pictures in iPhoto starts with staying inside iPhoto. Don't try to do ANYTHING with the pictures in iPhoto using the Finder. EVER.