PHP does not have built-in image editing capability, but many projects use the GD library and the servers I work on have this already installed.
I am distributing an open source product, Camera Life, which lets people manage their photos in a website they run (like Gallery). In there I need to create image thumbnails. Users are running somewhat recent (5.3+) PHP on Mac/Linux/Windows with Apache/IIS or whatever else is popular.
For a stable and modern PHP project, is it best practice to abstract out the image processing functions or hardcode in routines that assume the user has GD installed?