I have an iPad 3 that I recently restored to factory settings and set up as a new iPad. With no photos on the photos app, it shows no data when I look at the usage list in settings. When I added one photo it jumped to 14.5Mb. When a second photo was added, it jumped to 16.5Mb. If both photos were about 2 Mb each, that leaves 12.5Mb for other data. If your photos app is still showing 100Mb, there must be something there that escaped deletion. Are you sure there aren't any folders in the menu bar at the top of the screen when you open the photos app?