progressive JPEG 
Unlike normal JPEG images, which are displayed one line at a time from top to bottom, progressive JPEGs are displayed in alternating lines, then filled in on a second pass (similar to GIFs). Depending on which Web browser is being used, progressive JPEGs may produce a venetian-blind effect or simply a blurry or blocky image that gradually sharpens. Pages using progressive JPEGs let people see at least the outline of an image sooner (helpful for users with slow modem connections to the Internet) and often appear to load faster than pages using normal JPEGs. If a browser does not support progressive JPEGs, it simply displays images as normal JPEGs.