If it was synced from your computer then it can't be deleted directly on the iPad, you will need remove/de-select it from where you synced it from on your computer and re-sync.
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