What is resolution? What does it really mean?
Resolution refers to the clarity and crispness of an image, which is measured in points or pixels per metric unit (inch, centimeter, etc.). Pixels are units of a single color and value that make up an image. The more pixels or dots in a set area, the smaller the dots are, the finer the detail and the higher the resolution. If the resolution is low, that means there are fewer pixels per inch, which means each pixel is automatically made larger. When the resolution is very low, you can actually see the blocky pixels — that's where the term "pixilated" originates.
A pixel's size is dependent on the size of the image and in relation to the density of pixels. Consider this scenario: two images of the exact same size are divided into squares. Each square can only represent one color and value and together they will be used to display the image. The first image is divided into 300 squares, and the second into 150 squares. As a result, the 300 squares in the first image are smaller than the 150 squares in the second image. Comparing this to resolution, the first image has a higher resolution than the second image because it has a greater number of smaller pixels.
However, resolution is not a set thing; it can change when the image size changes. Using the example above, imagine taking the first, high resolution image and enlarging the entire image to twice its original size. The number of squares, or pixels, remains the same (300), but the pixels themselves become larger to fill the larger area. It terms of resolution, this new larger format has lowered the resolution of the image. So what this means to you is that a high resolution image at one size can become a low resolution image at a larger size. Make sure that your images are high resolution at the size you want them to appear in your book.