What do the marks on the small pictures in the Designs column mean?

The number at the top left is the design's place in the book. A lock means the page is locked. "AI" means the page was drawn by your image engine, "Photo" that it was made from your photo. A yellow warning mark at the bottom right means the page failed the print check: point at it to read why. A spinning circle means the page is being made, and an AI page without a drawing shows its idea and "not drawn yet", or "AI page: no idea yet".