You can add new graphic elements to you mod by placing .jpg and .png images into the "graphics" folder of your mod and the subfolders it contains. For some purposes (such as objects), the editor can access all images anywhere in the graphics folders but for other purposes (such as surfaces and edges), only the images in specific subfolders are available.
Note that both the images from your mod's graphics folders and the mainmod's graphics folders are accessible from your mod.
terrain subfolder: Contains the images that can be painted on the surface of terrain elements and polygon-type objects. They are also used to define the floor types that can be painted on the ground. However, simply having an image in this folder is not enough to be able to paint this image on the ground, you also need to create a corresponding terrain type in the terrains.ini file in the "script" folder of your mod.
edges subfolder: Contains the images that can be painted on the sides of terrain elements and the polygon-type objects. Images starting with _ only cover the side of an object and not the edge of the top part.
Useful sources for images:
Wikimedia Commons (http://en.wikipedia.org/wiki/Wikimedia_Commons)
Most images on the wikimedia commons website can be used, modified and incorporated into other works as long as you credit the original author of the artwork. Some are fully public domain and can be used without crediting.