[Update: significantly improved the Perl script]
The hardest part of my cunning plan isn’t “making a screensaver”; I think every OS comes with a simple image-collage module that will do that. The fun was in collecting the images and cleaning them up for use.
[Update: turns out it’s easy to extract previews for the “look inside” style; just open the thumbnail in a new window, and replace everything after the first “.” with “_SS500_.jpg”.]
A bit of clicking around assembled a set of 121 pleasant images at more-or-less the same scale, with only one problem: large white borders. I wanted to crop these out, but a simple pass through ImageMagick’s convert -trim filter would recompress the JPEGs, reducing the quality. You can trim off almost all of the border losslessly with jpegtran -crop, but it doesn’t auto-crop; you have to give it precise sizes and offsets.
So I used both:
crop=$(convert -trim $f -format "%wx%h%X%Y" info:-)
jpegtran -crop $crop $f > crop/$f
These were not, in fact, generated by taking screenshots of the screensaver. It takes a long time for it to fill up all the blank spots on the screen, so I wrote a small Perl script that uses the GD library to create a static collage using the full set of images. If I desaturated the results a bit, it would make a rather lively background screen. For home use.