No reason to transfer the image data in JSON, just read the response as an ArrayBuffer. Pass over that and write it into a dataURI with the needed modifications in a single go.
How are you going to do that? There's other things in the payload. If it was just the image in the response you could read it as binary, but remember we want lots of tiny images coupled with titles and other metadata. You can use something other than JSON but it'll need some kind of structure that needs parsing.