ImageMagic is a collection of useful command line tools that allows you to edit an image without using a heavyweight photo editing application like Photoshop or GIMP. This is great for batch processing a large volume of images. Here are a few tasks that I found useful for my own uses:
To rotate an image:
convert -rotate <degree> <src file> <target file>
To resize an image:
convert -geometry 800x600 <src file> <target file>
To add watermark:
composite -dissolve 50% -gravity SouthEast -geometry +5+5 <watermark file> <src file> <target file>
composite command deserves an additional comment. The -dissolve option defines how visible you want the watermark image to appear on your target image. -gravity option defines which side/corner you want to place the watermark in the target image. The choices are NorthWest, North, NorthEast, West, Center, East, SouthWest, South, SouthEast -geometry option allows you to define offset values on the placement after applied -gravity option. In the command above, the input values will cause the watermark image to move 5 units (first +5) left of the right boarder and 5 units (second +5) above the bottom border. So there is a little bit of space between the boarder of the target image and the border of the watermark.
See official web site for more information.