You can only delete photos on the ipad that were taken with the ipad camera, otherwise you have to use itunes on a computer.
iOS and iPod: Syncing photos using iTunes
iPad Tip: How to Delete Photos from Your iPad in the Photos App
How to Delete Photos from iPad
How to: Batch Delete Photos on the iPad
(With iOS 5.1, use 2 fingers)
Any images synched to the iPad with iTunes (from a computer) can only be removed via a synch. Images downloaded through other means, or taken with the iPad camera app, can be deleted directly from the iPad.