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.

<IMG 
   SRC="../graphics/paulonbike.gif" 
   LOWSRC="../graphics/paulonbike_lowsrc.gif" 
   HEIGHT=378 WIDTH=516 ALT="A bike and his boy">

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