Helping myself remember how to do common things using imagemagick’s (excellent but many-optioned) convert utility.
convert -scale 10% {in} {out}
# convert to black and white
convert -type Grayscale {in} {out}
convert -monochrome {in} {out}
# invert colours
convert -negate in out
convert -rotate {in} {out}
# make the given colour (e.g. here white) transparent
convert -transparent white {in} {out}
# make transparent white
convert -fill white -opaque none {in} {out}
Make square (for thumbnailing)
convert -background transparent -gravity center -extent 145x145 file1 file2