I just ran the same test with no strange image proxy. I would check on your APN settings, they may be set to WAP internet which may use some sort of image compression service. Also, pull up an image directly on your device and see if you get the same results.
And yes, a carrier could figure out you were tethering, if they really wanted to. While there was at one point a "no tethering" clause for Android on T-Mobie, I have never heard of any instance where they enforced it. If you're a user that consistently comes close to the BW cap every month it may be something to worry about. If you stay below I feel safe to say you're in the clear.