Icons - are different from standard images. They are have standard sizes (usually small): 16×16, 32×32, 48×48… One icon is composed of several images. Each of them with a different size and number of colors (mono, 16 colors, 256 colors, 16.8M…). And the most important property of an icon is the ability to include transparent areas. This allows seeing the screen background behind the icon within its square image.

Image - A computer image is a bitmap (composed of pixels) or vector (composed of drawing paths) picture, which can be saved using various formats (BMP, PSD, GIF, JPEG, WMF…). All these formats have several different properties (bitmaps, vectors, compressed, layered, animated…) and can be used to store pictures at any sizes and resolution.