Hi Bob,
welcome to the forum and good luck with the "side" business. The images you have on your website are very nice.

Marketing is the name of the game and you have a good start. There are essentially two audiences for you to reach:
1. the end-user: the folks who have the pets you wish to photograph
2. the influencers: the folks who work with those other folks.

You have already started reaching out to the end-users. The influencers are groomers, veterinarians, trainers, etc. You live in a very pet-centric part of the country so you'll find a lot of people in the market for your product. Don't forget the role of pricing in the marketing mix. PetSmart, PetCo and others sell pet portraits for virtually nothing. That will be some of your competition.

Again, welcome and we look forward to seeing more of your work.

Jim