The small boxes containing a red "X" indicates that any kind of "<IMG" tag did not have its content loaded. It doesn't indicate any one graphic type; it could be .gif, jpg, .tif, whatever.
If you are seeing the missing graphic indicators consistently on one browser and not seeing them for the same pages on another browser, the browser that shows them probably has its option to load graphics disabled. This is something people with limiting bandwidth modems frequently do in order to load pages faster.
Check the help files for your browser to find where the option is located and enable it again.
Russ