When the hard disk is quickly run out of space, you could delete multiple files by plugging the nexus 7 into your computer. It's also other way to do it right on the handheld.
How to Delete Mulitple Photos on nexus 7
* Open the Gallery app and navigate to the album folder which locates the photos you want to delete.
* From the preview screen, touch menu key on bottom and tap "Select" item.
* Select photos you wish to delete and long hold on the images. A long menu will pops out and you need to select the delete option or tap the dustbin icon.
If you want to remove Photo from Gallery, go into the gallery and Press on the photo you want to delete and hold your finger down and then it will ask if you want to delete.