Hint... if your image link doesn't end it .jpg, .png, or some other image type it won't work. I do not know if it is possible to get such a link from dropbox. Better to include the images as attachments even though that might require extra posts.
I tried opening one of your links by copying and pasting the URL into a browser window. Permissions error. Not surprising. Dropbox isn't intended for this kind of sharing.