well you might want to check out the usleep() function @ http://www.php.net/usleep
but I think it might not help.
I'm not sure what your trying to do, but your logic might be flawed.
If you want it to display an image while the script executes - it won't work - because PHP runs server side and the script finishes running BEFORE it exports to the browser.
which means if you echo an image and then have PHP do stuff -- the image wont display until the entire script finishes running.
your alternative would be to either use AJAX - load the image then, call the script to load on another page in the background and replace the image w/ its results - this can be done with javascipt (google AJAX)
or, use two pages. the first one has the image load then a meta tag or something forwards to the page w/ the script. the first page will hang, displaying the image, for as long as it takes the second page to finish processing. then it will load the second page