thanks for replying. I guess I've been overly optimistic – a stream of hex RGBA is not really an image (e.g. I'm not even sending width&height, just the raw data).
I've been looking at this page, and apparently prepending
'data:image/bmp;base64,' to my Base64 stream isn't enough, because it misses a lot of header tags... My goal was to avoid saving on disk a temp PNG to read it back (Photoshop API allows me that), but perhaps it's the safer way...