Example: Optimising Images for Quality

Using GIMP we took a photo with plenty of detail, cropped and scaled it down and saved it in JPEG format at three different quality settings: 100, 88 and 25.

Quality 100 88 25
Result Highest quality photo of tortoise Medium quality photo has slightly less detail but still looks OK Lowest quality photo shows even less detail but could still be acceptable on a web page
Close up Close up of high quality photo demonstrates highest accuracy Close up of medium quality photo demonstrates less accuracy Close up of low quality photo demonstrates lowest accuracy
Size (kB) 19.8 6.2 3.39
Time at 20kbps 7.9s 2.5s 1.4s

You can see the trade-off between quality and file size pretty easily over this wide range, in the loss of detail, colour accuracy and rectangular artifacts that become apparent in JPEG images at high compression levels, such as the tortoise's head being reduced to a couple of coloured blocks. But in many cases this doesn't actually matter very much, and a high compression level can be used without problems.