Based on Dan Kubb's suggestion, I’ve implemented an ActiveResource-style accessor for rest-client. This also supports basic auth, so now:
r = RestClient::Resource.new('http://example.com/photos/', user, pass)
r.put File.read('pic.jpg'), :content_type => 'image/jpg'
The static methods for raw URL access are still available as well. See the updated RDocs for details.