Bill the Lizard answered an image compression question on StackOverflow by pointing out the image below that shows the difference between PNG and JPEG compression when applied to line drawings.
The image comes from lbrandy.com. The left side of the image uses PNG compression, a lossless compression format. The right side uses JPEG, a lossy format that computes a Fourier transform and discards the highest frequency components. JPEG can produce smaller files for natural photographic images. But for line drawings, the artifacts of the JPEG compression are noticeable.