Tried to recreate and the Facebook app just ended up crashing on my iPhone 4...
Let's say you were about to select the nth image. If you go to the photo app and delete, say, the (n - x)th image, then go back to the facebook app, selecting the nth image will actually select the (n + 1)th image. If you select the _last_ image, it will crash (presumably with an array index out of bounds.)