A few basic (but powerful) ImageMagick commands

Image credit: ImageMagick
convert image.jpg -resize 1024x768 output_file.jpg
convert image.jpg -resize 1024x768! output_file.jpg
convert image.jpg -resize 1024x768\! output_file.jpg
convert image.jpg -quality 75 output_file.jpg
montage image1.jpg image2.jpg image3.jpg image4.jpg output_montage.jpg
montage image1.jpg image2.jpg image3.jpg image4.jpg -geometry +2+2 output_montage.jpg

Another way to create a collage of images

montage -mode concatenate -tile 1x image1.jpg image2.jpg output_montage.jpg
convert output_montage.jpg output_montage.pdf
convert pdf1.pdf pdf2.pdf output_pdf.pdf
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=temp.pdf pdf1.pdf pdf2.pdf

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