I had the same problem. I don't know why I even tried this but it seems to have worked.
After double-clicking the file I wanted to view and having it show up behind the window I clicked from, I noticed that the Photo Viewer was 'Maximized' to fill up my screen. I double-clicked the top window bar to 'Restore Down' which will show the window like it had been manually sized and placed where wanted on the screen. Doing that caused the window to become the top layer window as usual. Then, instead of using the red 'X' in the top right of the window to close it, I clicked on the top left of the window to bring up the window menu and clicked 'Close' from there. I double-clicked the exact same file and it suddenly opened up on top of all other windows. I closed it the same way and tested it again. It opened up again on top. I opened it and closed it using the red 'X'. It opened up again on top. So, after opening it once again, I double-clicked the top window bar to 'Maximize' the window to fill up the screen and closed using the top left window menu 'Close'. I opened the same file yet again and it opened on top of everything else as a maximized window.
I don't know enough about this stuff to figure out WHY it worked, but I thought I'd share the fact that it did work for me. Give it a shot.
I'm the third in a row that don't know much about this but I was reading where Microsoft wasn't putting somthing to do with the photo viewer in windows 7 but was available via free download. I would use the term photo viewer very loosly as I really don't recall the exact name of the feature.