I needed to do batch operations with images and get a merged PDF output. Desiring to minimize a number of tools, at the end I’ve done all processing with ImageMagick and Ghostscript. Zsh and GNU parallel were also my friends.
Convert and Chop 4 sides
convert input.png -crop +300+300 -crop -300-300 output.pdf
Convert and Rotate
convert input.png -rotate -90 -page A4 output.pdf
Convert many files in parallel
find . -name '*.jpg' | parallel convert {} {.}.pdf
Merge and optimize
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -sOutputFile=merged.pdf \
one.pdf two.pdf three.pdf
Result file will be optimized because of -dPDFSETTINGS=/ebook.