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convert image.png -quality 95 image.jpg

will try to preserve the aspect ratio, but the image may not be exactly 200×100

convert example.png -resize 200×100 example.png

to force the image to become a specific size – even if it messes up the aspect ratio:

convert example.png -resize 200×100! example.png

height of 100:

convert example.png -resize x100 example.png

rotating an image:

convert image.jpg -rotate 90 image-rotated.jpg

batch processing

for file in *.png; do convert $file-rotate 90 rotated-$file; done