aah ok, I can help you with that :-) So you use the following format under your photo
source
Inside the square brackets you type whatever text you want to see in the clickable link. Then, don't leave any space between the square brackets and round brackets and then inside the round brackets copy past the URL of where the photo appears on its own on the free photo library that you have used. I have literally just written the word source between 2 square brackets, then opened up a pair of round brackets and popped the word URL in. You would copy the URL link from the photo library site that takes you directly to the photo you are using. Try that and let me know if it works for you :-)