If you are assigned the task of capturing computer screenshots and images for your computer related book like a technical manual or a how-to guide book, follow these instructions from Cafepress:
You can capture JPG or PNG high resolution screenshot images using any screen capture program like SnagIt or use the Print Screen key on your desktop.
1. If you save using JPG format, try using the lowest compression setting in Photoshop. Lower compression means less distortion. If you have the choice of having no compression at all that would be ideal.
2. When creating images, start your image at 200-300 DPI. You can always decrease your image and maintain a clean, crisp image.
3. The recommended image format for saving screenshots for books is either PNG or JPG.
a. PNG is a completely loss-less compression. Gradients come out much smoother and do not have the distortions that may appear in a JPG.
b. JPG supports over 16 million colors, but slightly "distort" the image to compress the file size. Link.