Only photos taken with the iPad, copied onto it via the camera connection kit, or saved from emails/websites etc can be deleted directly on the iPad - either via the trashcan icon in the top right corner if viewing the photo in full screen, or via the icon of the box with the arrow coming out of it in thumbnail view. Photos that were synced from your computer are deleted by moving/removing/de-selecting them from where they were synced from on your computer and then re-syncing.
Acually King Penguin I think this works in Photo not iPhoto on the ipad. All my comments refer to iPhoto on the iPad, not iPhoto on a laptop or desktop MAC. I find iPhoto on the iPad to be very unfinished application. As near as I can tell when you make an edit in iPhoto it creates a copy of your photo that it puts who knows where. If you actually want an album that you can get out of the black hole called iPhoto you have to save those photos in camera role. the steps are:
1, Switch to edit mode
2. Select the photo to be 'deleted' (actually hidden not deleted)
3 . Touch the X icon on the bottom of the screen and that will hde the photo
Easy enough but why not use the same well chosen tools used in Photos? Why make it different so that you have to experiment or refer to the non-existent "iPhoto on iPad manual"?
But now comes the real fun part. Suppose you have edited a number of photos and hidden a bunch more and you want an album of the editied and un-hidden photos to use as you see fit. In my case I am totally unintersted in creating a Flicker account. I don't want to tweet them or facebook them. I want to get them out of the iPhoto black hole. Here are the steps I found. I would be thrilled if someone has found a better way. What I found is multistep very slow process for each photo to be added to "Camera Roll". Camera Roll is the way to get Photos accessable in Photo and thereby available outside the black hole.
1.Go to Edit mode (upper right corener)
2. Select All Photos (upper left corner)
3. Choose a photo out of the photo matrix.
4. touch the Share icon. that's the one that looks like the select icon in Photos (bad choice Apple. i kept thinking I mucst be doing something wrong since it sort of looked like select bu wasn't).
5. Touch Camera Roll
6. Touch Selected
7. Wait. This is a slow process
8. Touch done
Repeat steps 3 through 8 for each idividual photo.
I gave up after 13 out of 60 photos. Way, way too slow. it would have take over an hour to do this six step process for the 60 photos. Instead load the photos from the flash card in my PC. Use Photoshop or Picasa and forget using m iPad for on the fly editing and showing photos which is what I had hoped to do.
The links below have instructions for deleting photos.
iOS and iPod: Syncing photos using iTunes
iPad Tip: How to Delete Photos from Your iPad in the Photos App
Another Way to Quickly Delete Photos from Your iPad (Mac Only)
How to Delete Photos from iPad
How to: Batch Delete Photos on the iPad
(With iOS 5.1, use 2 fingers)
How to Delete Photos from iCloud’s Photo Stream