Cool ImageMagick stuff

Make thumbnails with ImageMagick's convert utility using:
convert -resize 25x25% filename.png filename-thumb.png

The convert utility can also convert image formats:
convert file.jpg file.png

Another cool tool in ImageMagicks arsental which can convert images is mogrify - this one's more useful when there are a many images requiring conversion:
mogrify -format jpg *.png

This will annotate an image:
convert -font helvetica -fill white -pointsize 36 \
-draw 'text 10,14 "Garden, Pune"' \
img_314.jpg annotated_314.jpg

To make an image look like it was sketched:
convert -charcoal 2 \
portrait.jpg charcoal-portrait.jpg

To make images out of a pdf file:
convert bigfile.pdf bigfilepages-%03d.png

You can also combine effects:
/bin/convert -size 460x90 xc:transparent \
-font AvantGarde-DemiOblique -pointsize 72\
-draw "text 10,75 'AvantGarde'" \
-fill darkblue -stroke yellow \
-draw "text 15,70 AvantGarde" test.jpg

This results in an image that looks like