you are making this WAYYYYY harder than it needs to be.
first of all, you can include as many query string or get variables as you want. just follow the format ?id=xxx&id2=xxx&id3=xxx
so what you really could do is simply
$id = $_GET['id'];
$ext = '.php';
if(strpos($id, $ext)){
include($id);
}
else{
echo "<img src=\"path/to/mydefautlimage.jpg\">";
//or echo "<img src=\"$id\">"; if you want to pass the name of the jpg dynamically
}
if you insist on including a php file and have it return a jpg, you would need it to use the gd libraries to build the image and output it.