image description ->post_content

ok so im working on a site…
http://davidmolnarphotography.com/blog/

i want do display the image description in the rollover at the top of the page.

i have accomplished this using this <?php echo $all_photo_arr[0]->post_content; ?>which shows the 1st image desc
or <?php echo $photo->post_content; ?> which shows the last img desc.

but i want it to change when the images change.

so i added this to the js that switches the images.

//switch image
function SwitchImage(img){
	$jpreloader.fadeIn("fast"); //show preloader
	var theNewImg = new Image();

	theNewImg.onload = CreateDelegate(theNewImg, theNewImg_onload);
	theNewImg.src = img;

	document.getElementById("info").innerHTML= theNewImg.src ;  

}

this shows the new image src. i have tried replacing it with the above php echo, but it only loads the 1st or last desc.

it seems i need to replace the .src with somthing. any ideas?