So you got that cool background image in your Gliffy diagram you want to start decorating with shapes and text. But, as you start dropping things onto the image and moving them around, you accidentally move the background image around too. Now you have to re-position it again. This can be painful and annoying and may cause swelling of the brain.
Thankfully there is Lock Shape. Yes, Lock Shape. Just select that background image or any shape you want to lock into place and click the Lock Shape check box in the right-side properties area of Gliffy. The shape overlay will now have red handles, signifying the shape is now locked. Yes, no more re-positioning those background shapes and wantless Undo-ing.
(Warning: Lock shape can cause extreme excitement and spontaneous ogling at your diagram)