King Penguin states it so well and deserves the credit ....
Photos that were 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. Tap the trashcan icon in the top right corner if you are viewing the photo in full screen, or tap the icon of the box with the arrow coming out of it in thumbnail view.
Photos that were synced from your computer via iTunes can only be deleted by moving/removing/de-selecting them from where they were synced from and then re-syncing. If you want to delete all synced photos from the iPad then try just syncing an empty folder
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