After you have the concept of resampling under your belt, how do you know what size you should be resampling to? How many pixels do you need? Here are your general guidelines:
Photos for your inkjet printer:
Inkjet printers are stochastic printing devices: That is, they use a series of droplets to replicate each pixel in your image, as shown in Figure 2-12. In theory, the optimal image resolution is ƒ1?3 of the printer’s rated resolution.
For example, a printer rated at 720 ppi works best with images at 240 ppi.
1,440 ppi printer, the formula calls for an image resolution of 480 ppi (and that goes for the 2,880 x 1,440 printers, too). However, most folks find that regardless of the printer’s rating, they never need an image resolution higher than 300 ppi.
Web images: Ignore resolution. Ignore the entire Document Size area of the Image Size dialog box. Consider only the image’s pixel dimensions. Determine what area of the Web page the image will occupy and then resize to exactly those pixel dimensions.
Page layout programs and commercial printing: If your image is to be placed into a page layout program’s document and sent to a commercial printing facility, you need to know the line screen frequency (the resolution, so to speak) of the printing press on which the job will be run. Ask the print shop or the person handling the page layout. Your image resolution should be either exactly 1.5 times or exactly twice the line screen frequency. (You shouldn’t notice any difference in the final printed product with either resolution.)
PowerPoint presentations and word processing documents: Generally speaking, 72 ppi is appropriate for images that you place into a presentation or Word document. You should resize to the exact dimensions of the area on the page or slide that the image fills.

In this wonderful world of digital imaging, you see resolution used in four basic ways:
Image resolution: Image resolution is the size of your image’s individual pixels when you print. I go into greater detail about this concept in the upcoming section, “Picking an image resolution.”
Camera resolution: Digital cameras capture each image in a specific number of pixels. Check your camera’s user guide or open one of the images in Photoshop and choose Image? Image Size. Take a look at the number of pixels that your camera records for the width and for the height. Multiply the numbers together, divide by one million, and round off the result. (If you’re in the camera maker’s marketing department, make sure that you round up.) That’s the megapixel (MP) rating for the camera. Use it as a general guideline when shopping. If you create Web graphics or snapshot-size prints, 3 or 4 MP is fine. For large prints, you need at least 6 MP.
Monitor resolution: Monitor resolution determines how many pixels are visible on-screen. Whether you use a Mac or a PC, you set the monitor resolution at the system level (as shown in Figure 2-4). When you use a higher monitor resolution, you get a larger workspace, but each pixel is smaller, which might make some jobs tougher. Experiment to find a monitor resolution that works just right for you.
Mac: Click the blue Apple menu in the upper-left corner of the screen and choose System Preferences. Then click Displays. (Some Apple monitors have a button on the bezel that automatically opens the window for you.)
Windows: Choose Start?Control Panel and then double-click Display. Click the Settings tab to see what monitor resolutions are available.
Printer resolution: Unlike the three preceding terms, printer resolution doesn’t involve pixels. Rather, a printer resolution tells you how many tiny droplets of ink are sprayed on the paper. Remember that it takes several droplets to reproduce a single image pixel — you certainly don’t need an image resolution anywhere close to the printer’s resolution!
(See the following section for more on this.)