A lot of devices have this problem.
A real quick fix is to download another gallery.
Open the play store and find an app called quick pic.
After its done installing, use it to remove any unwanted pictures.
Then reboot.
If you don't just want to loose the photos plug your phone via usb into a computer and turn on mass storage by clicking the notification in your notifications slide.
Find your device by looking under "my computer" or "drives" now locate removable disk.
open it, locate DCIM And copy it to your favorite folder. Now turn off mass storage.
You should have backed up your photos.
Also you may need a larger MicroSD card if you want to store more photos.