When you create a Category in your custom library, you have the option to browse and select the icons for it.
By the way, in previous versions of Inventor, once you have created the new Category, you cannot change the icons you have selected, because the options are grayed out when you open the Category Properties dialog box.
This has been improved in Inventor 2014.
After installing the Update 1, you will be able edit the Category Properties of an existing category and change the icons you have selected when you have created it.
Besides, in case you have copied a standard Category Structure to your custom library, if you want, you can also change the icons for these Categories that will display in Content Center with the custom icon when the custom library is in use in the active project.