After you put the sig photo in your sig block like fishfreek said, you have to make sure to scroll down in your profile to "Preferences". Specify "yes" to the option "Always attach my signature". When I added my pic, it only added to posts I wrote afterwards. Previous posts didn't get the pic added. If you would happen to change your avatar, it adds to all posts, past and current, but the sig pic doesn't seem to add that way.