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.
If you're going to self-publish, do your research first! Almost all companies have some negative review, but sometimes if you just read them ahead of time you'll know what to watch out for! Make sure that you:
Post your portfolio online, in places where children's book authors look for illustrators! Remember that you can show only thumbnails, or images with a watermark, if you're worried about posting your artwork publicly online. Look online for advice that authors give, about finding illustrators, so you know what employers are looking for (like this article).
Read more about the process of creating and pitching a children's book here.