Just spent a couple of hours fixing a weird problem with inserting images that I thought others might find useful as I couldn't find a solution anywhere else.
Problem:
When trying to insert an image into the content field (tinymce), selecting the folder in the Image sidebar doesn't show the thumbnails - instead it just stays showing "(Choose a folder or search above)".
Solution:
Don't create a has_one relationship on your pages called Image ($has_one = array('Image'=>'Image')). The resulting ID clashes with the Image sidebar and so breaks the select javascript.