How do I increase the resolution of my images so CreateSpace won’t complain?

Resolution is basically the number of dots (for print) or pixels (for screen) per inch; if you want to increase the resolution from 72 to 240 or 300 dpi, then you can do that in some image editing software. However, if you don't at the same time make the image smaller then you will lose some quality in the image and it may not print as well.

In general, I ignore the CreateSpace warnings about image quality and just do my best to put an image that will print well into the document, and then tweak it if necessary after receiving a proof copy and I see how it actually looks.

Generally speaking, I'll figure out what the size of my image is going to be on the page, and will edit the image to be that size before inserting it into the document. For the sake of easy math, if I have a 720×720 pixel screenshot at 72 dpi, then it would take up about 10 square inches of real estate on my screen at actual zoom and the height and width would be the same as 10 inches by 10 inches. If I change the pixels per inch to 300, and want to maintain the same quality, then I can have up to a 2.4 by 2.4 inch image in the print book. Anything larger, and I'm potentially losing quality.

Many images will print just fine at a lower quality, however, so the best method is really to put the image into your document, ignore the warnings, and to order a physical proof to see what the image looks like when you are holding it in your hands. Even if you do have high resolution images, it isn't guaranteed to look good when you print it in black & white using Amazon's printers, so it's worth looking over every image carefully.