Microsoft Dynamics CRM 2013 : Add picture for a custom entity

To add a picture (16x16 and 32x32) for a new entity, go to “Settings / Customize the system” and select the target entity :

Click on “Update Icons”.  Click to choose the good web ressource who contains your .jpeg or .png

To create a new web ressource, click on “New” :

Add your picture to the web ressource, save and publish your record. Make the same operation for the 16x16 and the 32x32.
 
Select the two ressources and click on “OK”. Save and publish.
 
Your new picture is available in the main ribbon:
Hope this post helps you ! :)