You can just "copy" the link information from your photo sharing site and "paste" it into the message. You could also just type it out as well. You can always check to see if it works by clicking "preview post". That will show you what your post will look like before it is published and you can actually click on your link to make sure it goes to the right place. If it is correct make sure to then hit "submit reply". Hope that helps, good luck.