You can use content/blob APIs to upload images - as alternative. Of course it will require some code, but I don't know of better solution.
Regarding ServiceAPI - can you contact our developer support with more information. It should work, and if there are errors, the error messages should be clear.
What is the prefered way to manage entry assets using the service api? I have a bunch of product images that I need to upload and assign to the products. I started using the media service bulk operations but that is quite clumsy... And it stopped working after a while, error message that contentLink is missing. No clue about that.
Isn't there some other way to work with images?