I am pretty sure I understand the issue, since I was looking around for the same thing. Through trial and error, I found no way to choose a static image for the extended display.
There is a solution I found that involves the SLIDESHOW function, though. I picked a folder with a few pictures in it, and I told my Windows 8.1 to do a slideshow of that folder. It began with the second monitor looking bland, as usual. However, at the end
of the first picture on the main monitor, a picture appeared on my second monitor. From there, they were both going through a slideshow.
This did not simply duplicate the original monitor, which I was happy about. I am personally satisfied with this solution. It's not the ideal thing that we were all looking for, but it is better than nothing. Hope this helps. I'll add some pictures of my
experience.

Original issue Above.

Selected option above.

Proof of solution working above.
As I said, I'm sure not everyone will be satisfied. Just know that I understood the issue and tried for a solution. Though, my system is Windows 8.1. I cannot vouch for those still using Windows 7.