I hope this will help you.
This is how I changed mine so that I could see the thumbnails.
Click on "settings" (top corner of this page).
Look to the left at the menu and scroll down until you see "General Settings". Click on it.
Scroll down until you see "Miscellaneous Options".
Under this heading you will see " Message Editor Interface".
Click "Enhanced Interface".
Scroll down further and click the blue box that says "Save Changes"
Hope this will work for you.