Why do my desktop background pix keep defaulting back to Apple standard photos?

I have a late 2013 Macbook Pro Retina 13" and since it was new I keep setting up my desktop background on my various desktops with my own photos. Every time, the screens all change back to the standard Apple "wave" photo. Very frustrating. I have the same model in the 15" and have no problem with that one at all. Any help would be appreciated.