If you are talking about pictures of charts (not editable chart objects) and
if other images (jpg) still show up fine, I'd suppose you don't have write
access to the "Temporary Internet Files" folder (or it even doesn't exist).
Open the Windows registry, go to
"HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User
Shell Folders" and check the "Cache" value. You might want to set it to
something like "%USERPROFILE%\Local Settings\Temporary Internet Files".