When photos/illustrations are used to convey an idea before a final product is produced they're called Concept Art. "Concept art usually refers to world-building artwork used to inspire the development of media products" (Wikipedia).
Visual aids are among my favourite support tools for my writing. I can browse Pinterest for hours to find the perfect photo of a character or a place in the world. They don't always look exactly as I want them to, of course, but they are a good base to start from. Not only does it make it easier for any potential artist you may hire in the future, it also helps you keep track of what your characters wear and look like.
I post concept art of locations and characters in a Facebook group I made for this specific purpose (Taigeria Archives). Among the millions of marvelous photos and illustrations on Pinterest I found some that perfectly embodies a few of my characters.
This is Pien Kaberela, the very first librarian.
Look at all that magic and impressiveness :O To be fair, what I would like for myself most in this photo is the ability to levitate books. I don't need to be able to levitate anything else, just books.
This is Seraph, a vacation city that is part of the Academic Academy.
Much sun, many relax. A popular city among the people living on the Academy's island. Not only that, but the city was built for one of the Academy's founders, a member of the waterfolks, and so have the city is almost as large below the water as it is above.
This is Hamma Nightwing, a character you'll meet in the first book. ;) She's a captain in the royal guard of Hedalia. Nightwing is the title she received for her skills of infiltration.
Do join the group if you're interested of getting more sneak peeks, asking questions, and interacting with other people interested in the series!
I always recommend people to find concept art for their creative creations (if applicable). However, do remember that you can in no way shape or form take credit for these photos. A general rule of thumb is that you should always, if you can, use a link to the artist's work. If you can't find it (as is the case with these that I found on Pinterest), at the very minimum you should inform the viewers where you found it and that you are not the original owner of the piece. This is so very important. You are using this artist's work to further your own, the least you could do is to acknowledge it. You wouldn't want someone to take credit for your hard work, now would you? ;)