If I understand your request correctly, you are looking to upload an image from an iOS device to the cloud using Google Cloud Endpoints.
As far as I'm aware, Google Cloud Endpoints does not accept multipart/form-data encoding uploads so you would not be able to use such endpoints directly for uploading images or any file for that matter. The solution requiring the least amount of effort would be a small App Engine service that accepts uploads and processes them before saving to Google Cloud Storage. An example can be found in
Reading and Writing to Google Cloud Storage.
Depending on your application requirements and authentication requirements, you may also want to consider using Cloud Storage's JSON API directly from your iOS application. With the solution and uploading images (usually within 10MB), I would use a multipart upload for sending everything with a single request. An example of the core network requests required can be seen in
Performing a Multipart Upload. Since there's currently no client library specifically for Cloud Storage with iOS, you'd need to either use the generic
Client Library for Objective-C REST or handle
API authentication yourself.
Hope this is helpful.