ImageMagick

If you use a Linux machine, you have probably heard or used ImageMagick before. It is a program that lets you manipulate pictures, and convert them to about a hundred different formats.

You can also use it directly from the command line.

So if you had a directory full of jpeg images, say, which you wanted to resize, and convert to png, you could write a very simple shell script.

If you like GUIs, you could open an image by saying:

$ display pic1.jpg

and then interacting with the GUI (which looks like something from the 80s, but is quite powerful).

It also lets you take screenshots (partial or complete) with the least amount of effort.

$ import pic.jpg

Use the mouse to select the portion you want to capture, and it gets saved in "pic.jpg"