Warning! Please be advised of what you are doing when giving permission for portal administrators to upload skins.
This ability allows any of your administrators to upload server-side code, and there is no security checking of the uploaded skins.
If you do not explicitly trust the people with administrator privledges to have the same rights as a Host / SuperUser then you should not enable portals to upload skins.
If you are the only one running the site and all the portals then you should log in as Host / SuperUser and you will then be able to upload skins.