You upload an image to dropbox and get the share link of that image. If you have a very large image then the share link for that image doesn’t display the original image. It displays a low quality image.
Here is a trick to get the original image URL link.

When you get a call on your sony xperia phone (android) then it automatically replies the caller with a reject message.
One of my friends had this problem on Sony Xperia M. This problem might persists in other Sony Xperia phones as well. This is annoying because the message is sent unknowingly.