If you aren't able to use the Photos tab to remove them, then can you temporarily delete something else directly off the iPad first e.g. a large app, video ?
Synced photos can't be deleted directly on the iPad. You will need to re-connect the iPad to your computer and select it on the left-hand sidebar of your computer's iTunes, and then use the Photos tab on the right-hand side of iTunes to de-select the synced folders/albums that you no longer want on the iPad - if you want to remove all synced photos then select and sync just an empty folder.
In terms of Image Capture (and the same will apply to iPhoto and Aperture), they will only see photos that weren't synced from a computer
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