Currently it is possible to customize the Service Category icon only in the Enterprise Edition and this is an issue and will be corrected in a future hotfix where we will be making it available in Standard and Professional Edition and you may further track this with an Issue ID: SD-49758.
Maljeev
SDP Support - FAQ
ServiceDesk Plus - Help Desk Software of your Choice
Maljeev
SDP Support - FAQ
ServiceDesk Plus - Help Desk Software of your Choice