I have been using the Sharethis plugin on 2.5.1 with no problems at all. I have just installed WP 2.6 and the images now do not show, either on the posts or in the plugin editor in the admin panel. Instead, it just has little mini grey squares with a cross in the middle.
I have deleted and reinstalled as well as deactivated all other plugins but it does not solve the problem.
I have a similar issue with the PodPress plugin which will not display images in the plugin editor in the dashboard.
Please help as this really affects SEO and traffic building efforts!