Below are the examples Salesforce gives you for creating formula fields that will display images such as flags, and stoplights. At the bottom is a link for the full knowledge article.
Flags for Case Priority
This formula displays a green, yellow, or red flag image to indicate case priority.
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")
Color Squares for Case Age
This formula displays a 30 x 30 pixel image of a red, yellow, or green, depending on the value of a Case Age custom text field.
IF( Case_Age__c > 20,
IMAGE("/img/samples/color_red.gif", "red", 30, 30),
IF( Case_Age__c > 10,
IMAGE("/img/samples/color_yellow.gif", "yellow", 30, 30),
IMAGE("/img/samples/color_green.gif", "green", 30, 30),
))
Traffic Lights for Status
This formula displays a green, yellow, or red traffic light images to indicate status, using a custom picklist field called Project Status. Use this formula in list views and reports to create a “Status Summary” dashboard view.
IMAGE(
CASE(Project_Status__c,
"Green", "/img/samples/light_green.gif",
"Yellow", "/img/samples/light_yellow.gif",
"Red", "/img/samples/light_red.gif",
"/s.gif"),
"status color")
Stars for Ratings
This formula displays a set of one to five stars to indicate a rating or score.
IMAGE(
CASE(Rating__c,
"1", "/img/samples/stars_100.gif",
"2", "/img/samples/stars_200.gif",
"3", "/img/samples/stars_300.gif",
"4", "/img/samples/stars_400.gif",
"5", "/img/samples/stars_500.gif",
"/img/samples/stars_000.gif"),
"rating")
Consumer Reports™—Style Colored Circles for Ratings
This formula displays a colored circle to indicate a rating on a scale of one to five, where solid red is one, half red is two, black outline is three, half black is four, and solid black is five.
IMAGE(
CASE(Rating__c,
"1", "/img/samples/rating1.gif",
"2", "/img/samples/rating2.gif",
"3", "/img/samples/rating3.gif",
"4", "/img/samples/rating4.gif",
"5", "/img/samples/rating5.gif",
"/s.gif"),
"rating")
Horizontal Bars to Indicate Scoring
This formula displays a horizontal color bar (green on a white background) of a length that is proportional to a numeric score. In this example, the maximum length of the bar is 200 pixels.
IMAGE("/img/samples/color_green.gif", "green", 15, Industry_Score__c * 2) &
IMAGE("/s.gif", "white", 15,
200 - (Industry_Score__c * 2))
https://help.salesforce.com/apex/HTViewHelpDoc?id=useful_advanced_formulas.htm&language=en#ImageFormulasTitle
