If you want to update the image for a particular CI or service, you can configure it on the CI directly. For example if you use the console you can typically do it from Configuration Management > Service:
If you use WebAccess to manage your services then make sure you have the "Image" attribute added to your Service window and you will be able to do it from here.
If you want to set the icon on a category of the catalogue, you need first to add the "Image" attribute to the Configuration Management\Catalogue Hierarchy Category window using Window Manager. You will then be able to set the picture in the Service Catalogue Hierarchy window:
This link from the online help explains how the catalogue hierarchy works: https://help.landesk.com/docs/help/en_GB/LDSD/10.0/Default.htm#../Subsystems/SCUser/Content/CreateHierarchy.htm
You can also find more information in the Workspaces and Service Catalog guides available here: Documentation for Service Desk
Hope this helps