You can do this in multiple ways:
1) create an animated GIF
2) brake the progress bar into pieces and flush() a piece every second.
3) use javascript
he is an example
<?php
$piece = "#";
$times = 10;
for($i=0;$i<$times;$i++)
{
sleep(100);
echo $piece;
flush();
}
and here you'll have a scipt that outputs somethingevery 100,
you can replace the # with <img src=..... > and you'll have what you need, (I think )