Image upload is not supported, nor is uploading images via freebase.com. Unfortunately, there is some old documentation that still references the functionality of the defunct api. The upload endpoint you found is non-functional and it is giving a misleading error (it should be a 404) I will correct that. We're not supporting image upload primarily because of the difficulties that arise with copyrighted images which has been discussed on this thread in the past. I apologize for the confusion. That mql doc needs to be to corrected.
In the Freebase API documentation  there's a nice example on how to upload images to Freebase, although the example relies to a deprecated library .
From the library web site, there's a link to the new documentation on Google , which unfortunately does not include any information about the upload service, which appears to be running here (sand-box):