All good web designers know that you should use the right image file type for the job. GIF and PNG are good for pixelly type things, and JPEG is good for photoy type things. Something you have to look out for is that if you compress JPEGs too much, you get compression artifacts due to the lossy compression and it all looks a bit crap.
None of which explains how the FT managed to get JPEG compression artifacts on a GIF! (See it in context). Muppets!
Back when I was doing a lot of work with web designers and ad-agencies back in the late-90s, I saw this happen a lot. My guess is that some web designer created the design, sent a JPEG (complete with artifacts) to the HTML guy, who promptly cut the graphic into it's various parts and exported those parts as GIFs (probably to keep the K size down).
- Stevan
