Hints for Editing Images
Different programs can produce surprisingly divergent results when exporting pictures for the web. For example, for two thumbnail images of book covers I recently posted there was a 3x size difference depending on which program I used. One of the images took 1/3 the space when compressed using Preview.app, the other when using ImageMagick. It's good that I looked at the file sizes before posting, and knew to try another program.
If the image you are compressing is a PNG, try pngcrush. Depending on the image, you can usually save about 15% of the filesize.
When using a small image as a desktop wallpaper, it is often useful to expand the image background to fill the full screen. This script finds the background color of the original image, then expands it to the specified size. The shave statement is used for images with a one pixel border.
#!/bin/bash # get colour colour=`convert $1 -crop 1x1+1+1 txt:- | sed -n 's/.* \(#.*\) .*/\1/p'` # crop & expand mogrify -shave 1x1 -extent $2 -background $colour -gravity center $1