I've found it. I was right, one of your styles is different. You appear to have changed the name of the image in the EAA BLUE style, but if you're using the vBulletin default styling, you're still giving the old one.}
To make the problem happen go to SETTINGS...GENERAL SETTINGS and scroll down to the "FORUM SKIN" choice and switch to "Basic vBulletin Gray" style. That give the broken image link.
The "EAA Blue" gives the right one.