Images are files that specify how pixels have to be 'turned on' on the monitor's screen.
There are several graphic formats, such as GIF, JPEG, TIFF, BMP and others, but GIF and JPEG are the most used graphic formats on the web. So you have to think about these issues. You should use the ALT tag in order to show an alternative text in place of the image. In this case, a text based browser will show the alternative text. The alternative text will explain the image content.