If you have upgraded and now cannot insert an image using the rich text editor's Insert button, you may have a permissions issue.
Check the Network Service user's permissions on your site's <filestorage> folder. The user must have at least Modify permission (that is, either Modify or Full Control) to the folder in order to insert images.
Even if you originally applied this permission to the folder, you might have affected the folder permissions in the upgrade. Reassign the permission by simply removing Full Control and applying those changes. Re-apply Full Control to finish the reassignment.