If you use Vodafone's UMTS service in Germany, you have probably noticed that JPEG images are compressed in low-quality mode. This is done by a transparent HTTP proxy, and there is no obvious way around it. Apart from very poor image quality, you might see messages about IP address 1.2.3.4 in your browser's status bar, and pop-ups advising you to press Ctrl+F5 to improve image quality.

2x magnified original and re-compressed JPEG image

The software that does this is provided by Bytemobile, and is used by other phone companies.

For MS Windows, the Vodafone HighPerformance Client apparently allows you to change the compression settings, but if you use Linux, you are on your own.

Fortunately, it is pretty easy to convince the Bytemobile proxy to stop messing around with your images. All that is needed is contained in a small Perl script, which sends a magic byte sequence to the proxy. As long as the script is running, compression is turned off.

The magic bytes should be the same as the ones sent by the HighPerformance Client, but use the script at your own risk.