ImageMagick is a collection of tools to mozify images, mostly from the CommandLine. Capabilities include:

• Resize, rotate, sharpen, color reduce, or add special effects to an image
• Create a montage of image thumbnails
• Create a transparent image suitable for use on the Web
• Turn a group of images into a GIF animation sequence
• Create a composite image by combining several separate images
• Draw shapes or text on an image
• Decorate an image with a border or frame

Usage Examples

Resizing

convert -sample width x height input.jpg output.jpg

The widths/heights can also take % values. It scales proportionally, so going 25%x25% on a non-square image will still make it come out non-square

Joining images together

convert files +append output.jpg

The +append appears to join them together left to right. -append possibly does something else. Haven't really looked into it.

Situation: You have a bunch of digital photos that you want to string next to each other for a website banner

1. Resize them all to 400px high (Just for ease of doing stuff later)

for img in *.jpg do

convert -sample x400 "$img" "$img"

done

2. Join them together

convert *.jpg +append output.jpg

This will of course join them in alphabetical order. You may want to do something about that.

3. Final resize/add gradients in TheGIMP