Raster graphic definition: it means that photos are made of many small points or pixels. Pixel is a single point in a raster image. Raster graphics images are also called bitmaps.
You can see that here in enlarged photos. They are about 1500 % larger than photo "Autumn tree" below. And you can try it yourself too, just zoom (enlarge) your photo to 1500% and you'll see a lot of unrecognizable little squares.
You can find more about raster graphic here.
Gimp is free raster graphics editor, similar to Adobe Photoshop®.