SYMPTOM
Chart image is a small box with a red X on the web page when viewed remotely.
CAUSE
This happens because the temp directory path specified is a full path such as "c:/web/tempDir". When you right-click on the image > select properties, you can see the path the browser is looking for the image in. Unfortunately when a full path is specified in this way, it cannot be mapped to a web address that allows users to view the image remotely.
SOLUTION
To fix this issue, use a relative path:
TempDirectory = "images";
For more information see this tutorial.