As a side note I've also seen some ISP's engage in website "compression," particularly on mobile phones with low datacaps or older, slower internet sources like 56k dialup providers. These pre-load the page on their end, compress everything as best they can, and then send it to you for viewing; if your ISP does this, it will display images as though they were re-saved as a lower-quality jpeg.
To my knowledge this isn't a particularly common practice anymore thanks to widespread high-speed and less issues with datacaps, but it's still something to bear in mind if you're on a slower service level.