iPhoto accesses the same library as the iPad Pictures app.
As far as I can tell, you can't delete an image from within iPhoto.
You can however make changes to the images. I think it duplicates the originals, on the other hand it looks like the changes are saved on top of the original (because you can undo the changes and return to the original).
Unfortunately, I'm not sure about the above. Things aren't transparent nor clear, like most iApps. Recently I discovered a Photobox in my iPhoto app, no idea where that came from.
Perhaps I should read the manual...