This thread may be enlightening: Spark.Publish max length (63 characters)
Even if we get clever with our encoding, a 50k image is going to have a few percent overhead when we ASCII-armor it (don’t even think about uuencode or Base64 or similar! Those wouldn’t be “clever”), you’d still be looking at 15 minutes to send the image to the Particle cloud.
If I were doing it, I think I’d open a socket directly to a file service and burst my data that way… In fact - that’s a project for this weekend since I have large’ish chunks of ascii sensor data to move off the microSD. In theory, with a direct socket, there’s no reason you couldn’t stream that bitstream up some place in approximately realtime.