A picture speaks a thousand words. Use images in your records to immediately draw users attention to important aspects of the information and give them a nice UI experience. Consider the following screenshot. By looking at the priority flag users will immediately know which cases are of high priority.
All you need to do for this is to create a formula field on Case with the following formula.
IMAGE( CASE(Priority, "Low", "/img/samples/flag_green.gif", "Medium", "/img/samples/flag_yellow.gif", "High", "/img/samples/flag_red.gif", "/s.gif"), "Priority Flag" )
To view other images that Salesforce provides, and they can be used in formulas please visit URL http://help.salesforce.com/HTViewHelpDoc?id=useful_advanced_formulas_image_links.htm&language=en_US