When designing your photo book or other photo product, you might notice that some images are flagged for being of a low resolution.
This means that the image doesn't have enough information or pixels in it to be reproduced at that size and still look good, at that size.
Possible reasons for low resolution images can include a reduction in size due to editing, size reduction for file transfer, or might even not have been there to begin with because of a low resolution camera or scan.
While it's impossible to get resolution back that might have been lost or not there to begin with, there is ever improving software designed to try and "guess" what that missing information might have looked like.
Photoshop and other image editing suits have built in tools to try and enlarge images while minimising the effects of that lost information and calculating what the added pixels should look like based on their neighbouring pixels. But these days there are smarter image enlargement tools that use sophisticated machine learning processes that more closely resemble how our human visual cortex might "imagine" what that missing resolution looked like and try to recreate those pixels in that way when enlarging the image.
If you've got an image in your book that is being flagged as low resolution, you might want to try some of these tools to try and get a better looking result at enlarging the image. It will never be the same as it could have been had that resolution not been lost in the first place, but some of these tools, when they work well, can give a much better result that just leaving the image at a low resolution.
Here are some you might want to try:
Deep Image: https://deep-image.teonite.com/
Let's Enhance: https://letsenhance.io/
Big JPG: https://bigjpg.com/
Remember these can never know what those added pixels actually should have looked at, they can only make varying degrees of guesses. The best solution for a low resolution image is replacing it with a higher resolution one, either by getting a better copy from the source of the image, or choosing a different higher resolution image to take it's place. If these aren't options, then enlarging the image might help you get a better result.
NOTE: Resolution is NOT a metric of the overall quality of an image it is related to the number of pixels in the image. If an image with enough pixels is out of focus or in any other way of poor quality, it will not be flagged. Images are flagged for low resolution only.