LeRoy,
Next time that happens pres <ctrl> <shift> Q in Firefox while on the forum page. That will open the Network monitor. Click the Network Monitor "Reload" button. It will then show you the download status of each element on the page. If you see an image with a RED indicator or 0 bytes size then that is an image that is supposed to be there but didn't make it for some reason. Pressing <shift> refresh often fixes it.