If there is an exclamation mark on picuter while design your project, it means low resolution picture. Low resolution pictures could cause blur or mosaic depands on what resolution it has. It is much better to replace with another picture or shrink to small picture size until low resolutino warning disappeared.
Usually printing requires 300dpi for images. Due to perform best quality of your photobooks, Photobook Designer will exam resolution of all pictures impoted in project. If picture is lower than 250dpi, exclamation mark will appear and this picture may be printed in best quality. The best way to result this situation is to replace with another picture or shrink to smaller picture size.
(Create Proofs to review Low Quality Pictures)
▼Printing results with different resolution