When scanning images for printing in papers, posters, etc., two important things to think about are pixels per inch & descreening. Pixels per inch is covered in detail on this page. Visit the page on descreening to learn more about it.
The first question to ask yourself when scanning images from a book, magazine or other print source is: How large do I want to print this image? Ideally, an image should have between 200 and 300 pixels per inch (ppi, also known as resolution or dpi) in order to be reproduced well.You're creating a poster for your thesis and you'd like to include an image of a painting. On your poster the reproduction of the painting will take up approximately a 3 x 5 " space. What size digital image would be ideal? Consider this calculation for an image that has a resolution of 250 ppi.
3" x 250 = 750 pixels
5" x 250 = 1250 pixels
You need an image which is at least 750 x 1250 pixels at 250 ppi.
Let's say you find a reproduction of this painting in a book, which is an 8 x 10 inch image. Scanning an 8 x 10 inch image at 250 ppi results in...
8" x 250 = 2000 pixels
10" x 250 = 2500 pixels
Great! This one will be more than big enough.
After scanning, open the image in photoshop and resize it to 750 x 1250. Or, leave the image larger than what you need (either exactly 2 or 4 times larger is ideal) and scale the image down within your document.
Remember: better to start with an image that is too big than too small, you can downsize later.