We first experienced this problem when we migrated from WebCT to Blackboard SP8. We are now on BB April 2014 release and no longer experience this issue. We resolved it by adding students to the permissions of the image file in Course Files. The easiest approach would be to ensure instructors create a folder in Course Files for all images that will be used and add students to have read access to that folder.
We also recall issues with images being attached instead of embedded in the text editor.
Hope that helps