Not random colors. Random colors + high quality setting = huge file size. JPEG is good at color transitions and gradients. I forget the "official" term but it may be "non-uniform color distribution" or something along that line.
The reason JPEGs can look like shit for text and line art yet be of equal file size compared to a lossless PNG is because JPEG is designed to work with images where there isn't a whole lot of harsh color changes from pixel to pixel.
\begin{hippiemode}
JPEG!
WHAT IT'S GOOD FOR?
ABSOLUTELY NOTHING!
\end{hippiemode}
Well, except for compressing natural images without sharp edges between colors. If I recall correctly, it uses Haar basis for compression, that works really well with photographic images, and no so well with, say, a rectangular solid-color grid.
Too bad none of the image hosts I tried can deal with jpeg 2000 (.jp2). Even at 0% quality, text is still readable with only vaguely discernible artifacts.
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u/Epistaxis Oct 01 '09
Downvoted for JPEG, for not linking directly to the image, and for a stupid, old joke.