Since I couldn’t get the mini_magick ruby gem to work (file not found aka the ‘identify’ command couldn’t recognize jpeg files for some strange reason), i resorted to good ‘ole xargs:

find . -maxdepth 1 -name ‘*.jpg’ -print0 | \
xargs -0 -I file gm convert -crop 1x1+0+0 file TOPCOLOUR/file