Below are two photos linked to a single file in my server. The first one you see should be in its highest quality while the second one should be exactly the same as the first IF YOUR INTERNET SERVICE PROVIDER DOES NOT DEGRADE IMAGE QUALITY by routing the image to a bandwidth optimization server. If you happen to see the second image to be inferior in quality, then the ISP you are using is doing something to save on bandwidth.
Here is another test reference:
Pay attention to details like the edges of the tree and the bird, also my signature. Check if you can see some pixelized portions and discolorations.
To resolve this issue, host your photos on services that offer Secure Socket Layer (SSL) for your site. This way, your photos will be encrypted while it goes from your host server to your viewer’s browser. This way, ISPs won’t be able to ‘touch’ or degrade your images so they can save on badwidth. So far, this is the only way I can think to circumvent this issue.
Hope this helps!