I don't see a problem. Image credits and URL can be placed beneath the image while editing the image capsule. These are not hyperlinks, and it has nothing to do with linking to an external domain as regards "overly promotional."
You can link to multiple domains, as long as you don't link to the same domain more than twice.
I use morguefile for most of mine, as they have an excellent selection, or flickr creative commons, but they do require attribution at the very least.
For all of these sites you need to read and follow their TOS on what they require in exchange for the free image. Some require nothing - morguefile don't for their free images, and even provide an image editor if you want to cut the image before posting it, some require attribution, and/or a link back to the site in exchange for the photo.
That is precisely why I gave you the suggestions that I just gave you. All images will be properly referenced, but only one hyperlink is needed - namely that beneath the first image, to which the others will be referring.
Image 1 Credit: "Example Artist A" URL: "ww..yoursource.com.." (a hyperlink)
Image 2 Credit: "Example Artist B" URL: "See Image 1 above" (no hyperlink)
Image 3 Credit: "Example Artist A" URL: "See Image 1 above" (no hyperlink)
I wouldn't worry about it. I believe that HubPages will know how to distinguish these photo sites from other websites. I do not expect that you will be flagged as overly promotional, because the moderators can see that you are linking to a photo site. In the unlikely event that you got flagged, simply contact HubPages, explain that the links are necessary to give image credits, and ask them to remove the flags.
Well, I use Flickr photos in every single one of my Hubs, without exception. Most of them have at least two photos. I hyperlink to the photographer's photostream every time. I have never had a problem.
I suspect HP must have some way of excluding the big photo sites from the overly promotional flag, or many Hubbers would be in trouble!
It's the same with YouTube videos, I often include 4 or 5 of my YouTube videos and I don't get flagged, because I can do a few drawing video tutorials in parts and these are needed to see the progression of a drawing, so with photos it could be the same thing, especially if you have a vested interest in displaying relevant photos.
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