Im a bit confused about your question because you mention invisionfree which I assume www.invisionfree.com
(assuming you mean www.invisionfree.com)
If you dont have the access rights (download and upload) to files in invisionfree, it aint possible. All you could ever do is follow what lxcid told you and use existing available uploaded skins on invisionfree

or request to the admin of invisionfree to install more skins. I dont know if invisionsfree allows you to edit come codes...but they have this service you could further edit your board, positions etc..
(assuming you mean your very own invisionboard in your account here)
You could those free invision resource websites like www.invisionize.com , download skins hacks, portals etc.. and follow instructions regarding uploading and altering files. A read me is usually included. Be aware of what version of Invision board you have.
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