How did the photos get onto the iPad ? Only photos taken with the iPad, copied onto it via the camera connection kit, or saved from emails/websites etc can be deleted directly on the iPad - either via the trashcan icon in the top right corner if viewing the photo in full screen, or via the icon of the box with the arrow coming out of it (Edit button on iOS 6) in thumbnail view.
Photos that were synced from your computer are deleted by moving/removing/de-selecting them from where they were synced from on your computer and then re-syncing.
There are third-party photo management apps such as Photo Manager Pro that allow more control over your photos e.g. you can delete photos synced to it directly in the app, you can change the photo sort order in the app etc
If you've copied them then yes - if they were taken with the iPad, copied to it via the camera connection kit or saved from emails/websites then you can copy those to your computer for backup purposes : http://support.apple.com/kb/HT4083
If they are photos that you synced from your computer then you will need to de-select them on the Photos tab when connected to your computer's iTunes and re-sync and they should be deleted from the Photos app - if you want to delete all synced photos from the Photos app then you could just select and sync an empty folder on that tab.
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
Thanks, Texas.....two of the methods didn't work , so they must be 'protected' photos.....but this is getting so complicated, I think my best bet is to connect to the iMac, delete everything, and resync the whole lot.
By the way, King Penguin, that Photo Pro app was no help in that the entire photo library was not an option when importing. Thus one had to look at every separate roll, which would take weeks.
Thanks anyway, gentlemen. Much appreciated.