This rotates, resizes, changes file format, grayscales the input image:

convert|mogrify -rotate 90 -geometry x760 -colorspace Gray -separate -average "$f" ["${f%.*}.png"]

convert writes a new file
mogrify is in-place

Opaquify an image with transparency:
convert -transparent-color acolor input.img output.img

convert from color to grayscale
convert -colorspace gray input.img output.img

tranparity an image
convert -transparent bgcolor input.img output.img