Interesting!!! This raises up some possible causes with the version of http protocol. I have to experiment...

Meanwhile, changing the header lines to...

<META HTTP-EQUIV="pragma" CONTENT="no-store,no-cache">
<META HTTP-EQUIV="cache-control" CONTENT="no-store,no-cache,must-revalidate,max-age=-1">
<META HTTP-EQUIV="expires" CONTENT="-1">

...did not produce any noticeable difference. The test image display_blank.png reloads properly, repeatedly, while the empeg_screen.png does not.

function RefreshScreen()
{
document.images.namedisplay.src = "images/display_blank.png";
document.images.namedisplay.src = "proc/empeg_screen.png";
setTimeout('RefreshScreen()',5000);
}

A snoop on the network shows:

Using device /dev/hme (promiscuous mode)
client -> empeg HTTP GET / HTTP/1.1
client -> empeg HTTP GET /images/default.css HTTP/1.1
client -> empeg HTTP GET /images/fascia_left.gif HTTP/1.1
client -> empeg HTTP GET /images/fascia_top.gif HTTP/1.1
client -> empeg HTTP GET /images/display_smoke.png HTTP/1.1
client -> empeg HTTP GET /images/fascia_right.gif HTTP/1.1
client -> empeg HTTP GET /proc/empeg_screen.png HTTP/1.1
client -> empeg HTTP GET /images/fascia_bottom.gif HTTP/1.1
client -> empeg HTTP GET /images/display_blank.png HTTP/1.1
client -> empeg HTTP GET /images/display_blank.png HTTP/1.1
client -> empeg HTTP GET /images/display_blank.png HTTP/1.1
client -> empeg HTTP GET /images/display_blank.png HTTP/1.1