Start your story as close to the inciting event as possible. In a picture book, with less than 32 pages, you do not have a lot of time. Yes, you do need to set up your hero/world but the quicker you can do it the better. I would shoot for one page, three pages tops.
Here are a few examples from picture books both old and new:
Oliver Jeffers's debut 'How to Catch a Star' - On page one we know what this boy 'WANTS' and by page two he has already begun his quest to obtain the object he desires.
William Steig’s Caldecott Medal winning book (One of my all time favorites!!!) 'Sylvester and The Magic Pebble' - We get a two sentence description of our hero, his family and his favorite hobby. Then BANG! He finds the magic pebble - inciting event.
A longer example comes from Master storyteller Dr. Suess and his classic ‘The Cat in the Hat.’ Here we get a super cute raining day opening that introduces the children and their boredom for about three pages then KNOCK! KNOCK! KNOCK! The cat has arrived!
Finally, Moe Willems does it on page ONE of his book ‘That is Not a Good Idea’ with an incredible Splash page of the Fox and the Hen making eye contact on the street.
I think some British guy said "brevity is the soul of wit.” In picture books you don’t have a lot of time; get in as late as possible. Trust me, your audience is super smart. They don’t need much for a set up, they will get it. And if you can tell them everything with a single image even better.