If you erase your iPad (Settings - General - Reset - Erase all Content and Settings), the images will be removed automatically, and your iPad will be set back to default.
Deleting all photos without erasing the iPad, works this way:
In the Photos app, go to the Photos tab, make sure you're seeing the collection, not year. There, tap "Select" at the upper right:
For every day, there will appear "Select" at the right. Tap this.
All of the images for the day you selected will be checkmarked. After tapping every "Select", tap the Trash button to remove the images:
That done, go to the Albums tab, choose the Recently Deleted album, and remove the photos from there as well, or else they can be recovered.