IIS URL Rewrite redirect to image to track email views

I am trying to build an email tracker based on the folder name that requests the image.

Example:

https://www.example.com/image/123/spager.gif

which need to be turned into this

https://www.example.com/image/index.php?id=123

which in return would serve the spacer.gif image.

Is this doable? And if yes, what am I missing?

So fare I got this:

web.config:

<rule name="email image tracker">
  <match url="^/image/([0-9]+)/spacer.gif">
  <action type="Rewrite" url="/image/index.php?id={R:1}" />
</rule> 

index.php:

<?php
  header('Content-type: image/gif');
  $png_image = imagecreate(150, 150);
  imagecolorallocate($png_image, 15, 142, 210);
?>

but I am getting the error:

The page cannot be displayed because an internal server error has occurred.

and nothing in the log files that would help me get a hint on what is not working.

I had the config file inside the image folder, so dropped image as part of the match.

Also <match ... /> was missing /> at the end.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="email_image_tracker" stopProcessing="true">
                    <match url="^([0-9]+)/spacer.gif" />
                    <action type="Rewrite" url="/i/index.php?id={R:1}" />
                </rule>
            </rules>
        </rewrite>
    </system.webServer>
</configuration>