Today I posted my update to ctshryock/MGTwitterEngine which allows you to use the API's
update_profile_background_image. I added to the demo app a window that lets you select an image for either profile image or background image to see it in action.
I had to modify some of the internals to get this to work... originally the
_sendRequestWithMethod:path:queryParameters:body:requestType:responseType: method built the
NSMutableRequest object internally, but to minimize code duplication I split the initial creation of the request into a new method
_baseRequestWithMethod:path:queryParameters: to do that, and added a new method
_sendDataRequestWithMethod:path:queryParameters:filePath:body:requestType:responseType: . Both
_baseRequest... to start, with the latter adding the needed form/multipart data to the request.