There are many graphics file formats in general use. Some have advantages over others for specific purposes. ImageGlue can save out images in a variety of formats.
Some types of graphics file formats can contain multiple images. For instance Photoshop documents often consist of multiple transparent layers each of which contains an image. TIFF (Tagged Image File Format) files may contain multiple images at different sizes and resolutions for different types of output.
Because image data is so large, it is often compressed. There are a number of compression methods in use but these may be more generally classified as lossy or lossless.
Although images generally have pixels, which are approximately 1/72th of an inch square, this is not always the case. The reason this value is chosen is because it is the same size as the dots on your computer monitor. However when you deal with other display technology this is not always the case. For example printers may typically be able to display 600 dots per inch. So for print work a number of image formats allow you to hold images which are specified at different resolutions.
Most file formats only support the visible portions of images. However some formats support transparency. Within a GIF image you can specify one color as transparent - essentially giving you a black and white alpha channel. PNG images can contain full alpha channels but while ImageGlue supports this many web browsers do not display such images as transparent. Individual layers of Photoshop documents may also contain alpha channels.