If the photos were placed onto your iPad using an iTunes synch operation, then they need to be deleted from the PC first of all and then you need to resync. If they were put onto your iPad without the use of iTunes - say you received them as an email attachment or took a screen shot - then there will be an option to delete the photo in the iPad Photos app.
Tim is correct...although if you want to keep them on your computer but take them off of your iPad you can sync specific photos by using the "Selected albums..." option under your iPad's "Photos" tab in iTunes. (Or maybe that's what he meant.)
I keep separate folders in iPhoto specifically for pictures I want to sync with my iPad...makes it easier to add and delete them when I sync up and doesn't affect the actual/original photos in iPhoto.