Some small publishers let you submit a book for consideration to be published, but you'll definitely have more success if you have a literary agent, especially with larger publishing companies. Read this article to learn more about literary agents.
Cast a wide net by searching for publishers online who have published similar books (photo books publishers, children's book publishers, etc.), and then look for their submission pages.
You can also consider self-publishing. Here is an article about the benefits of self-publishing for artists, and one about the risks of self-publishing.
If you're going to self-publish, do your research first! Almost all companies have some negative reviews, but sometimes if you just read them ahead of time you'll know what to watch out for! Make sure that you:
- retain rights to all your work
- get an ISBN for your book
- know what their contract says
- get proofs for pages and a guarantee for error-free printing
- avoid predatory or "vanity" publishers by searching for them on Better Business Bureau and reading reviews
You can also publish eBooks! Here are some resources to check out: