Recently I set Gmail to show as HTML to speed reception on a slow network.
When I attempted to return to "Standard View" navigation icons were missing, but images in the emails appeared normal. I cleared all cookies and the cache per the help.
Further checking revealed that the headers of other websites were no longer correct, (eg. TPAPS.com)
Attempts to correct the problem at the browser (Firefox) level were unsuccessful, including deleting and re-installing Firefox.
Checking other browsers, I found the same problem occurring in Internet Explorer. However, both Chrome and Safari displayed the pages properly.
Operating system is XP-SP3 on a Dell "Vostro" laptop with plentiful free memory.