I finally have it working now. For anyone trying to upload local images with C# (without the eBay SDK): Upload your whole workload (including the xml part) using bytes with a Stream. I tried to do it as a string with a StreamWriter and encoded the image bytes with base64, which is not possible, as it seems.
@shulmani_developer.support: Could you or anybody else from your team please have a look at this? I am simply not getting it done! Btw if I try it in production environment it gives me error code 2191454: "Uploaded picture has an unsupported file format."