In that case I think you have joined the group of us with broken screenshots.
You can take them using the Mac method of pressing cmd-shift-3 and then the images will appear on your desktop. Not ideal but it does work. The only trouble I find is that the pictures are very large, so I change edit them in Preview and convert them to jpeg to make the file size smaller.
Yes, that can be annoying. I usually pause the game if I want to take a picture, so I can try to spin it round and try to get the least annoying angle. I also press tab to get rid of the UI bar at the bottom while I take the picture.
It was much better when hitting button C worked and the screenshots appeared in the right folder. It's something I wish they would fix - it worked fine before the Generations patch so they broke something and haven't repaired it.