You might backing up all your projects to the cloud using the sync feature, and then try uninstalling and reinstalling Photoshop Touch.
Please double check also your free storage on the device. It could be that you need to free up some more.
how to restart an application on iOS:
1. double tap the home button -> leads to bar showing up with active applications
2. Select PS touch and touch the red/white cross icon next to it's icon.
Did you solve your problem btw? Did it always happen with one image or did it occur with all your images?