Image Magick

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.