ImageMagick and GD are 2 of the most popular graphic software libraries available on the market. They make the dynamic creation and editing of images and graphs on a site doable, so they need to be available on a web server if you host your 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 - create thumbnails, change the colors, flip it, blur it, etcetera. Needless to say, these modifications are doable if the script application which you use on your website permits you to perform such things. The 2 libraries could be used with different programming languages - PHP, Python, Perl, and so on., and support more than one hundred frequently used image formats which you'll be able to work with - PNG, PSD, JPEG, TIFF, GIF and many others.