I do it by highlighting the photo in Dropbox, select the paper-clip and copy to clipboard. Then in the email text area, tap and select paste. I am sure there are other ways but this one works.
Good luck.
Hi, if your jailbroken check out Any Attach in cydia, it does exactly what your wanting to do. It ads a paper clip to the email program. Simply click on it and send any type of file from your iPad, camera roll and drop box
hope this helps a little.
Use the link icon to Copy Link to Clipboard. Go to Mail and paste the link into your email.
There is no direct way to add the file as a full attachment in the Mail app (without jailbreaking), though you could use Open In to place a copy in GoodReader, then mail the attachment from there.