Hello Peter !

You need to perform the following actions:

1) go to the Studio->Storage->Images area, press the "Add" button, in the appeared box you need to upload your icon, then press "Done".

2) copy the URL value of the newly added record. It has a look like [UNA site URL]/s/sys_images/ltx9hmuvregraw5np42z3fvnxncbqrky.png

3) go to the Studio->[Your template name]->Styles->Custom Styles area of your mix and add the following lines to the end:

.sys-icon.my_new_icon:before, .fa-my_new_icon :before {

  content: URL("[your URL from the previous step]");

}

Save your changes.

4) go to the Studio->Ads->Categories area and edit the necessary category. In the "Icon" field (like in your screenshot) you need to set the value "my_new_icon" from point 3. Save it.