Presuming you want use the awesomeness of ImageProcessor (included in 7.2.0), here are some hints on getting this working with Umbraco 7.2.0 (RC) anyone that might be struggling.

Open your PM Console in Visual Studio and go;

PM> Install-Package ImageProcessor.Web.Config

This will update your ~/web.config and will add several new files in ~/config/imageprocessor/

Then browse to ~/config/imageprocessor/security.config.

Open it and ensure it has the following remote image service section;

 <service prefix="remote.axd" name="RemoteImageService" type="ImageProcessor.Web.Services.RemoteImageService, ImageProcessor.Web">
  <settings>
    <setting key="AllowRemoteDownloads" value="true" />
    <setting key="MaxBytes" value="4194304"/>
    <setting key="Timeout" value="3000"/>
    <setting key="RemotePrefix" value="/remote.axd"/>
  </settings>
  <whitelist>
    <add url="https://i.vimeocdn.com/"/>
  </whitelist>
</service>

Here I added the keys AllowRemoteDownloads and RemotePrefix. I also had to <whitelist /> the CDN I wanted to use.

Then, over in my view;

<img src="~/[email protected]?mode=crop&width=600" />

(where Model.Video.ImageUrl was a string something like https://i.vimeocdn.com/video/474774013_1280.jpg)

That should do it.

You then need to buy @jamesmsouth a beer.