Are you sure Photoshop Touch has access to your iPad's picture gallery? It needs to be granted access to your Camera Roll to save it there.
If not, this may be a bug. Try this:
- Make sure you have enough memory (you should have at least between 500MB and 1GB) available on your iPad Mini. If not, you may need to delete some apps, files, photos etc.
- Make sure you don't have too many apps running in the background. Double-push the home (square) button and close all apps that are opened/running in the background by swiping them "up". Photoshop can be pretty heavy on resources so there should be as few apps running in the background as possible.
- Try doing a "hard reset". Push the home button while pushing the lock/unlock button (on the top right corner) and keep holding the buttons down (simultaneously) until the whole screen turns black and you see your iPad restart with the Apple logo. This will basically "reboot" your iPad while emptying all its cache memory etc. By default, iOS uses a fast booting option to start the iPad pretty fast, but by doing so it keeps old stuff in its memory. Don't worry, you won't lose any files/app etc. (it just restarts the iPad).
- If this still doesn't work, unfortunately, you will have to uninstall the app, then do a hard reset, then download Photoshop again.
Good luck,