Most basic usage (convert file type)
convert SRC DEST
convert -format FORMAT SRC DEST # explicitly define type instead of relying on extensions
Resize image
convert -resize 50% SRC DEST # by percentage
convert -resize 64x64 SRC DEST # to precise dimensions
convert -resize 64 SRC DEST # fit to set height
convert -resize x64 SRC DEST # fit to set width
Rotate image (by degrees)
convert -rotate -90 SRC DEST
Negate colors (invert)
convert -negate SRC DEST
Difference between two images
composite SRC1 SRC2 -compose difference DEST
Compress a jpg (good quality/size tradeoff)
convert -strip -interlace Plane -gaussian-blur 0.05 -quality 85% SRC DEST