mogrify

mogrify - transform an image or sequence of images

• Convert between image formats (although convert, also part of ImageMagick, is specifically intended for that). All the image formats you've ever heard of are likely supported, though outnumbered by the ones you've never heard of.
• Reduce the number of colours in an image (colour quantisation)
• Dither colours.
• Blur or sharpen an image.
• Scale an image.
• Crop an image.
• Flip and flop an image.
• Add an annotation to an image.