Rewrite Subfolder to Subdomain in web.config

Question

I'm attempting to write a rewrite rule for the following scenario.

User attempts to load this picture:

domain.com/images/folder/picture.jpg

and instead, I need it to load:

cdn.domain.com/images/folder/picture.jpg.

Here's what I have that isn't working:

<rule name="CDN rewrite for Images">
    <match url="(.*)" />
    <conditions>
        <add input="{HTTP_HOST}" pattern="domain.com" />
        <add input="{REQUEST_URI}" pattern="^/images/folder/(.*)$" />
    </conditions>
    <action type="Rewrite" url="cdn.domain.com/images/folder/{C:1}" />
</rule>

UPDATE: Adding additional info. Most pictures are being served up from Joomla so while the root of the domain is something like domain.com, most images are input with a src="/images/folder/picture.jpg" Not quite sure how this is affecting the rewrite, but none of the options on cheesemacfly's answer below, are working...

UPDATE2: While cheesemacfly was unable to help me in my particular circumstances, I awarded him the bounty and marked his answer as the accepted one because he went above and beyond to try to help me in chat. Hopefully his answer will help someone with rewrites on IIS.

Answer 1

EDIT:

To be able to rewrite (and not only redirect) urls to outside websites, you need to install the Application Request Routing module and enable the proxy mode.

To do so:

1. Download and install the module
2. Open your IIS management console (inetmgr)
3. Select Your server node
4. Double click on Application Request Routing Cache:
5. Click on Server Proxy Settings on the Actions pane (right of the screen)
6. Check the box Enable proxy and click on Apply

The second step is about setting up your rules.

If you want your rewrite to be based on the path then use the following code:

<rewrite>
  <rules>
    <rule name="Rewrite to cdn domain">
      <match url="^images/folder/(.+)$" />
      <action type="Rewrite" url="http://cdn.domain.com/images/folder/{R:1}" />
    </rule>
   </rules>
</rewrite>

Or if you keep the same folder architecture on the second website you can simplify as follow:

<rewrite>
  <rules>
    <rule name="Rewrite to cdn domain">
      <match url="^images/folder/(.+)$" />
      <action type="Rewrite" url="http://cdn.domain.com/{R:0}" />
    </rule>
   </rules>
</rewrite>

If you want to catch only the files ending with a specific extension (let's say images):

<rewrite>
  <rules>
    <rule name="Forward to cdn domain">
      <match url="^images/folder/.+\.(?:jpg|bmp|gif)$" />
      <action type="Rewrite" url="http://cdn.domain.com/{R:0}" />
    </rule>
  </rules>
</rewrite>

Please refer to: http://www.iis.net/learn/extensions/url-rewrite-module/iis-url-rewriting-and-aspnet-routing (section "Which Option Should You Use?")

TIP:

The best way to test your pattern is to use the IIS test pattern tool.

At the root of your website -> URL Rewrite -> Create a blank rule -> click on test pattern:

If you don't get the expected result, you can debug your rewrite using the Failed Request Tracing tool