HTML Attribute for <IMG ...>
LOWSRC

Because some image files take so long to download, LOWSRC was created to download a smaller image first before the "main" image.

This Code Makes This 
<IMG 
 SRC="/graphics/paulonbike.gif" 
LOWSRC="/graphics/paulonbike_lowsrc.gif" 
 HEIGHT=378 WIDTH=516 ALT="A bike and his boy">
the image on this page

If your images are so large that you need to use LOWSRC, you probably should consider using smaller images.

<IMG NATURALSIZEFLAG="..."> >