It depends on the site, but Twitter for example will leave a file untouched if its heuristics decide the file is already well compressed. Good compression is expensive, and in many cases more so than the bandwidth you'd save.
Can confirm. I've been stuffing configuration files inside images and storing them on Twitter for a couple years now; they remain untouched, and any newly-booted VM on my home network can pull the files down via API, strip the payload out of the image, put it into place as a config file, and cycle the service, no problems. Works great. All of this was born out of an idea to get something 'useful' out of twitter, rather than "perpetual doom scrolling to find something to be upset about". Now I never touch their GUI, and they are essentially an offsite Puppet repository for me at this point.
I can also confirm that Facebook strips everything out of images, rendering them useless for this purpose. Instagram does the same (not surprisingly).
I know it's cliche to say here on HN - but this kind of comment is why I love this site. Silly, unexpected ways to use technology are so satisfying to me.
