ImageMagick is built on Ghostscript which i have introduced in the following post previously.
Ghostscript – Convert PDF Cover/Page into PNG

I am working on my Rails project with the Paperclip gem which requires ImageMagick. Installation of ImageMagick in Ubuntu is very simple. Just grab it by apt-get.

• apt-get install imagemagick

After installation, you could make use of the convert command to manipulate images.

# Convert png to jpg
convert kit.png kit.jpg

# Resize a png and convert it to jpg
convert kit.png -resize 50% kit-resized.jpg

Checkout all the available options of the convert command @ ImageMagick Official Site