Why does Amazon reject an A+ image?

A+ rejections are almost always about prohibited content, not the image itself. Amazon does not allow A+ content to include:

• Prices or discount/savings claims ("only $9.99", "50% off").
• Review quotes, star ratings, or "bestseller" claims.
• Time-sensitive or shipping claims ("new", "on sale now", "ships free").
• Contact information or links off Amazon (your website, email, social).
• Guarantee or warranty language Amazon restricts.

Keep your A+ images to describing the book — what it's about, who it's for, your other titles — with no prices, reviews, or promotional claims, and it will pass. See Can A+ images include prices, reviews, or promotional claims?