You can add credit/source for your image/video on the 'Quick Edit' screen or the 'Advanced Editing' screen. Here are the steps:
Step 1: Select the image/video for which you want to give credits.
Step 2: On the right panel, type the credits in the box that says, "Photo/Video credits."
Step 3: Press Enter and customize it the way you like it.
Here is a short video explaining how to add credit for your images or videos.