ImageMagick and GD are 2 of the most popular graphic software libraries on the market. They make the dynamic creation and editing of images and graphs on a site doable, so they need to be present on a server if you host your internet sites on it and you want to have something different than static images with fixed dimensions. If ImageMagick and GD are installed, you'll be able to edit any image in many different ways - generate thumbnails, change the colors, flip it, blur it, etcetera. Of course, these adjustments are doable if the script app that you employ on your website enables you to carry out such things. The 2 libraries can easily be used with a lot of website development languages - PHP, Python, Perl, etc., and support over a hundred commonly used image formats that you will be able to work with - PNG, PSD, JPEG, TIFF, GIF and many others.