When you are creating your book and you are placing photo's on your page, or even a two page spread and A warning appears on the picture, This does not always mean that your picture is of Low Resoltuion.
To find out what the warning means you can simply Right click on the warning box, then scroll down to the bottom where it says Show Warnings
It will then tell you what is wrong with your picture:
Object Overlaps Middle of Page
An image or text frame is placed over the middle of the book and that some parts of this object may be lost in the binding of the finished book. Please check that all critical elements of the photo (eg. faces) and all text be kept away from this area.
This is a warning message advising you to visually check the placement of the object - no action needs to be taken if you can see that no critical elements (eg. text or faces) are near the middle of the page
To fix what the warning message is describing you need to adjust the picture / text box to sit on either the left or right hand side of the page:
This is the text overlapping the middle of the book:
This is the corrected version:
*All warnings on our software do not necessarily need to be fixed if what you had created is the look you are going for. The warnings are purely there for your notification in case you were not intending to have your picture, overlapping the centre of the book or over the bleed edge.