Using System Preferences to set desktop

I have a Mac Pro running, 10.10.2 Yosemite. I set my desktop using iphoto at first, then I was forced to move my Iphoto library to an external hard drive, that is running and usb attached all the time. The library works fine except now I cannot chose Iphoto to access the photos to set my desktop, its grayed out. I then chose two photos I wanted to use (I have two monitors) and I pulled them out of Iphoto and just put them on the drive, I can and did chose them, and could see them. Then I restarted my computer and they do not show up at all. When I go into System Prefs, and check each monitor, they show the two photos, but they do not show up on the monitors in the small areas within system prefs, but not on each actual monitor. I must be missing a step of some sort. It's not an important issue, but really, this should work.

Can anyone help?

Thanks!