"Read Only Text" field controls have an option for "Do not submit". This option prevents the parameter from being shown in the submitted service request, but still displaying in the Service Catalog so that you could use it (for example) as a user text tip.
This article explains how to use that same functionality for something like an image or other control type.
In any other control type, you can enter CurrentUserRole()=="SelfService" in the Visible Expression section. This will make the parameter visible to SelfService roles but prevent it from being seen by any other role. It will be hidden, and effectively appear the same in functionality as the "Do not submit" option.