You can directly link to images using the IMG tag, but AFAIK they have to be placed in your Dropbox/Public directory first. Then just right click -> Dropbox -> Copy Public Link. This should copy the image URL to clipboard, which you can then paste.
edit: Actually, it seems like all you need to do to be able to direct link to images is to change the www.dropbox to dl.dropbox. So, you don't need to place them in the Public folder first.