You can use the GET methods of PHP.
If you use a url such as: example.com/viewer.php?imagePath=/images/myfile.jpg
Then the file "viewer.php" will be passed the value "/images/myfile.jpg" in the GET variable "imagePath".
To access this in your PHP code, you need to use $_GET['imagePath']. _GET is an array containing all of the GET variables passed in to the page.
So to include the image in your page, you would do something like:
echo "<img src='$_GET[imagePath]' />";
One thing to think about with using GET is that anyone using your site can type anything they want as a URL, so you might want to do some validation on your GET variables before using them, to avoid code injection attacks.