This is a lovely watercolour of a pair of geckos (Thanks, Alice!), scanned to deliberately show the paper texture. The problem is, that if your artwork looks like this, then it may be difficult to reproduce.
If we wanted to print this image across a whole page, then we need some space at each edge to 'bleed' off the edge of the page. The bleed is trimmed off after printing. This bleed is shown by the dotted line above (exaggerated).
As you can see, because important details of the image go right up to the edge of the paper, they could be lost 'off the page'.
One solution, is for you to use a high-key (white) background, because that is easy for us to extend. However, this image has a textured paper (which for a book, is generally good to avoid anyway) and part of the blue 'shading' goes right to the edge of the paper. This makes it difficult to extend the background.
This image shows the resulting page once printed. As you can see, after the printer trims the book, the image looks unbalanced, and important details have been lost.
The best solution if to ensure your images have plenty of 'quiet space' around them.
This image shows the quiet area around the geckos that allows us to adopt a variety of crops. The dotted lines show two possible crops.
Quiet space does not have to be blank, it must just be capable of being all or partially cropped without affecting the image.
Generally, on an A4 original, you should allow no less than 20mm of quiet space around the image.