Append images vertical:
convert -append IN*jpg OUT.jpg
Append images horizontal:
convert +append IN*jpg OUT.jpg
Trim whitespace:
convert IN.jpg -trim +repage OUT.jpg
Convert, force resize image ratio:
convert -geometry 1024x768\! IN.jpg OUT.jpg
Convert to BW with seperation + most green:
convert -separate -poly "0.25,1, 0.5,1, 0.25,1" IN.jpg OUT.jpg
Convert pdf to jpg:
convert -density 300 -trim IN.pdf -quality 100 OUT.jpg
Will create n jpgs (if there are n>1 page in the PDF), named OUT-0.jpg OUT-1.jpg etc.