IMAGE LIBRARY
Image library appears when clicking on the button of a material's texture parameter. Here you can store and manage images to be used in your projects.
On the top of the window you can choose which library type to handle:
project images: images stored in current project. They will be saved together with it: there's no need to distribute image files together with project files;
default material library images: they can't be modified and they're used by material library distributed with the software. Feel free to use them in your projects;
user material library images: images stored in your computer, not depending on a particular project. Please note that whenever you use an image of this library type (or the previous), it will be copied to "project images", i.e. stored in current project.
"Add" and "Remove" buttons are used to modify (when possible) current image library.