you would have to create a new provider for the Sitefinity's Images&Documents module and inside of it you would have to work with Sharepoint library as opposed to database as our standard provider does.
You can learn more about provider model from here:
You can then use Reflector
to see our implementation of the Telerik.Libraries.Data.DefaultProvider (default provider for Images & Documents module) and implement the provider accordingly for the Sharepoint.
I hope you'll find this information helpful. Let us know if there is anything else we can do for you.
the Telerik team