On the pix, you will find the info for how they want their linkback posted when you use their pix.
You can type something like this on the photo capsule:
Image source from pixabay.com/users name
then in the URL box, put the exact URL leading to the page the pix is on, not the tab you opened to get to the pix but the page where all the author info attribution info is.
the tab you opened to view it usually has no author info on it, so that's not the one HP wants you to use. They want the page you opened that has just that pix on it but also with author info so when quality control looks at your hub, they will see all the pix are properly attributed with workable links leading exactly to the correct place.