Have you tried the Image Allow Insecure Derivatives module?
Provides a user interface to allow Drupal to bypass the token security check when generating image derivatives.
Drupal 7.20 comes with a security fix that changes all image derivative URLs generated by Drupal to append a token as a query string. ("Image derivatives" are copies of images which the Drupal Image module automatically creates based on configured image styles; for example, thumbnail, medium, large, etc.)
s an example, links that previously pointed to a URL like: /sites/default/files/styles/thumbnail/public/field/image/example.png
will now point to a URL like: /sites/default/files/styles/thumbnail/public/field/image/example.png?itok=zD_VaCaD
For more information on the security improvement, visit the release notes.
This is a fantastic security improvement, but in certain cases, has some ramifications. This module provides a configuration setting in order to toggle whether or not Drupal bypasses that token check.