Are you using the free version of Invision or a paid version?

As far as I know with the free version it isn't possible. Apart from using the actual tools in Admin CP for importing smilies, and uploading pips and rank things.

If you're using the paid version, however, you should be able to use and FTP client to do it, or again use the tools on the actual menu parts in the Admin CP.

For example, if you want to upload a smilie, go to Manage Emoticons and use the browse thingy on that page to upload a new one.