Sheila's advice is spot on. HubPages is not referring to just boot campers but everyone. Any photo which is from public domain simply gets the source name where you learned it was public domain. This is not to be confused with the website "publicdomainphotos dot com" which is site similar to Pixabay and other photo sites. Pixabay is not public domain. For anything you use from them or any other photo site, it is part of their TOS that you must attribute and/or provide a backlink to them.
HubPages prefers everyone to attribute pictures with a source URL and a photographer's name (if available). But, they never said it had to be a LIVE link.
HubPages has a limit of 3 URL's from the same site which can be used per hub. So if you used any one site for 3 photos, you have used up your 3 URL's. You wouldn't be able to insert the same URL link within your hub narrative, text capsules, links capsules, or further reading, etc in addition to your photo capsules,
The way to get around that is to make as few of the links LIVE as possible. You do this by strategically placing links in Source Name or Description box because neither will light up as a blue (LIVE) link, therefore not counted against you. You will get a notice at the top of your hub that they won't publish it if it has more than 3 URLs to same site and will highlight them to show you where they are. To get published, you have to limit it to the 3 LIVE links per site.
So, put URLs in the Source Name box or in Description box - neither will be a LIVE link and won't be counted against you, Any URL that does not light up in "blue" is not a Live link.
It is in your best interests to always list the site and photographer to avoid being slapped with a copyright infringement notice, or a violation of TOS of another site. I always like to know I'm in the right, before I go up against the big boys fighting TOS or DMCA. lol
P.S. I learned the above about URLs from Writer Fox. Anything he tells you in the forums, soak it up. He's a great teacher!