Do you used httaccess to redirect your hits from YOURSITE.com to
WWW.YOURSITE.COM? If so, you might confirm that your images paths include the "www".
I moved servers a while back and discovered that only a few users could not see images and that problem was because of the www. Most people, including myself had the httaccess work and we saw the images just fine but for some reason I will never understand I had a few that always got the red "x" of doom...
Perhaps your members could also be entering from the DOMAIN.COM or
WWW.DOMAIN.COM and the image issue only happens one way or the other?
It is a shot in the dark but it took me a long time to figure out so it might be worth a look...!
Good Luck!