What makes it so difficult is that you can't simply spit out the image in the middle of the page. In order to use the image, you'd have to make a SEPARATE call to the script, like so:
<img src="myscript.php?cmd=getImage" alt="The Image" border="0" />
Why? Because in order to display this image, you'd have to send the appropriate headers.
If you simply stored the path, you wouldn't have to set up your script to be run twice, and instead the browser would simply download the image from the URL stored in the session. Less overhead, less difficulty, etc.