Carry out the following steps to create a formula field to display Salesforce images to graphically show the priority of case records:

1. Click on the Setup gear icon in the top right-hand of the main Home page, as shown in the following screenshot:

2. Click the Setup option, as shown in the following screenshot:

3. Navigate to the Case customization setup page, by clicking the following: Objects and Fields | Object Manager | Case | Fields and Relationships.

Locate the Fields & Relationships section on the right of the page.

4. Click New.

We will be presented with the Step 1. Choose the field type page.

5. Select the Formula option.
6. Click Next.

We will be presented with the Step 2. Choose output type page.

7. Type Priority Graphic in the Field Label text box.
8. Click on the Field Name. When clicking out of the Field Label text box, the Field Name is automatically filled with the value Priority_Graphic.
9. Set the Formula Return Type as Text.
10. Click Next.

We will be presented with the Step 3. Enter formula page.

11. Paste the following code:

/*********************************************************** 

Priority Graphic for the Case, for High, Medium or Low. 

For high priority cases a red flag is displayed; for medium an orange is shown and for a low a green flag is shown. 

***********************************************************/ 

IMAGE  

(  

 IF( ISPICKVAL(Priority,"Low"), 

   "/img/samples/flag_green.gif",  

     IF( ISPICKVAL(Priority,"Medium"), 
       "/img/samples/flag_yellow.gif",  
         "/img/samples/flag_red.gif"  
       )  
       ), 
       "", 15, 15  
)

12. Optionally, enter the following in the Description field:

Field to display an image according to whether the Case Priority is Low, Medium, or High 

13. Optionally, enter the following in the Help Text field:

Field to display an image according to whether the Case Priority is Low, Medium, or High 

14. In the Blank Field Handling section, select the option Treat blank fields.
15. Click Next, as shown in the following screenshot:

We will be presented with the Step 4. Establish field-level security page.

16. Select the profiles to which you want to grant read access to this field via field-level security. The field will be hidden from all profiles if you do not add it to field-level security.
17. Click Next.

We will be presented with the Step 5. Add to page layouts page.

18. Select the page layouts that should include this field. The field will be added as the last field in the first two column section of these page layouts. The field will not appear on any pages if you do not select a layout.
19. Finally, click Save.