ajax question (forcing image refresh)

I have a function that calls a php page to rotate an image
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                 var myurl = "rotate.php?img="+document.getElementById('r_img').value+"&r_dir="+document.getElementById('r_dir').value;
         http.open("get", myurl, true);
         http.onreadystatechange = rotateIt;
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That php page overwrites the image sent to it , rotates it 90 degrees and returns the same path of the image (after its been rotated and saved), when i get this image i refresh the src of the image
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if (http.readyState == 4) {
       document.getElementById(document.getElementById('id_img').value).src=http.responseText;

  }
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What happens is that on the page the image shown is from the cach of the page (doesn't change) , if i delete the temporary files and refresh the page i see the new rotated image but i want it to be done without refreshing the page at all .
I guess one option is to rename the image when i rotate it , that could work but its a little complecated for me do do it now because all the images are stored in a database with some logic to their names so im hoping to find a simple solution that refreshes the cach of the image
Thanks!!