Now, if we're talking about a novel, very likely the marketing department has nabbed galleys with which to make an ARC.
If we're talking about a picture book, though, that does not happen. Galleys are not representative enough of the final book to make good marketing materials.
In many cases, a picture book will simply be put on an early enough production schedule so that marketing can use advances. But sometimes, they'll have the printer send over a bunch of F&Gs.
F&G stands for folded and gathered.
F&Gs are the same sheets the printer is sending the publisher as proofs, but instead of sending the printed sheets straight off the press, the printer has begun (but not finished) the process of putting the book together.
When putting a book together, after the printer has printed the sheets of paper that will go into the book, those sheets are folded where the paper will butt up against the binding, gathered into signatures, and bound.
The most expensive part of any hardcover book, though, is the binding, so for the purposes of marketing materials, they skip that last step.
What you end up with looks like this:
This is an F&G.