Did you use iTunes to put hte photos onto your iPad? If so, you need to use iTunes to take them off.
If you simply saved them to the camera roll then when you select the photo you will see a little trash can icon. tap on that to delete it. Or you can also, from the album view, select multiple images and then delete them.
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
iPhoto for iOS (iPad): Delete photos from iPhoto
How to: Batch Delete Photos on the iPad
(With iOS 5.1, use 2 fingers)
How to Delete Photos from iCloud’s Photo Stream
Delete Pictures from Your iPad