Except, if the photo has been put on the iPad by a synchronisation operation with your PC, then you can't delete it on the iPad. Instead you need to delete or move it on your PC or Mac first of all and then resychronise the iPad. Only photos that are on your iPad as a result of being received not via a synchronisation operation can be deleted directly on the iPad.
If this restriction proves too cumbersome, then there are many third party photographic apps available that don't have this problem.
Tim