What do the little exclamation icons in Photobook Designer mean?

This could mean any of the following below:

 
1) Low Resolution Warning: When this icon pops up on a picture, it means that the picture you’ve selected is too low-resolution to print clearly at the current size. You are recommended to either shrink the picture until the warning icon disappears or replace your picture with a higher-resolution picture.

2) Missing Image: This could also mean the software cannot locate your original photo file and you will need to import these images back into the software to relink them. If you have used any photos from a external drive or a CD, ensure that they are connected and that the drive letter remains the same at all times. (e.g. If your external drive was assigned the D drive when you imported the photos into the project, make sure it is always D drive when it is connected.