You are being warned that the photos are low resolution or the photo aspect ratio are incorrect, and that the final quality output (in the printed book) may not be up-to-snuff. However, the photos will still be there in the book (without the Red overlay).
Below are the Release details:
May 4th 2017 - Quality Assurance
We have made checking the quality of your books easier then ever! From Subject Management both you and your schools can quickly check to make sure your yearbook photos are up to snuff.
From the Composer - subjects and images will be highlighted if we detect any issues which need to be corrected. We also provide you with a Quality Report in the side tab for you to check that everything in your Book is in tip top shape and ready to print!
To ensure you have full control over the rendered quality of your Books the minimum DPI for subjects and candids can be controlled inside of your Studio/Lab Settings.