Posting photos from iPhoto to Facebook
It turns out that although you can't do it with CMD-C and CMD-V, you can drag and drop the thumbnail of a photo from iPhoto into Facebook. So I did it this way:


Find a photo I want to post
Press the space bar to go back to the thumbnail view. The thumbnail of the photo I was looking at is highlighted in yellow.
Click-and-drag the highlighted thumbnail and, while holding the mouse button down, press CMD-tab to switch to the browser
Drop the photo into Facebook
Repeat for other photos until the post is finished


It's not exactly ideal but it did work. I won't accept my own answer in case someone has a better way.