Display picture not working.

I set my dp (display picture) a while back, but logged in one day to find I
had been reverted to the default grey icon. I tried to set the dp I wanted
(tools>options>change picture>browse>) and selected the picture I wanted,
then pressed open. The browse window closed, but the dp stayed as the default
icon. I changed the image format from jpg to bmp, which worked for about a
day or two, but then reverted to the default grey icon.
Now neither the jpg nor the bmp work. However, every other photo in my
pictures library works. Can someone help me with this problem?
PS I have made sure "show my display picture and allow others to see it" is
selected in options>personal.