Well if nothing has changed (the picture files aren't bigger, you aren't editing them any more, etc.), it could be faulty RAM or other programs are using more than they previously were. RAM, in case you don't know, is your computer's memory. You might first consider if you are using more programs in the background. If you aren't using any more programs, you should test the RAM to see if it's faulty. Depending on the OS you are using, you might be able to run a test on bootup.