Prior to the last forum upgrade, closing a viewed photo was easy; just click on the "X" in the upper right-hand corner. Now, the "X" is gone and the viewer has to guess to hit the ESC (Escape) key to close the photo and return to browsing the thread.
My suggestion would be to either return the missing "X" or put a small message like "ESC to close" somewhere on the edge of the photo.
Chuck