Strange, I copied a site, which has a lot of img changing when mouse hover over
the image, with the following code :

img onMouseOver="src='i/h_home.gif'" onMouseOut="src='i/a_home.gif'"
alt='Home' src='i/a_home.gif'

the a_home.gif is copied, the h_home.gif is NOT copied

Both GIF's exist on the source !