How many pixels do you need in aimage?

This will depend on what you plan to do with it. Image resolution refers to the number of small squares, called pixels, that make up the image and provide its detail and colors. The greater the number of pixels an image contains, the larger the file size, and the larger the print you can make from it.

Quirky thing to know about:

In computer graphics, there are really two different types of resolution to be aware of. The first and most important is the image resolution mentioned above, measured in number of pixels per unit length in an image and is measured in ppi (pixels per inch)

But there is also the monitor that you're viewing the image on to consider. The number of pixels per unit length on a monitor is called monitor resolution and measured in dpi (dots per inch)

In day-to-day use, these units are used interchangeably, but they are not the same and can cause some confusion when your image resolution is higher than your monitor resolution. In this case, the image will appear larger on screen than it’s specified print dimensions. A "pixel dense" image may appear HUGE on your monitor, whereas it will print out looking normal.

This is because in Photoshop, image pixels are translated into directly into monitor pixels, therefore, a 1 inch by 1 inch image at 144 ppi will display as a 2 inch by 2 inch image on a 72 dpi monitor. It will print as 1 by 1, but display larger. If you’re working with images greater than 72 ppi, it’s a good idea to display rulers and or check the print size to monitor the ?real? dimensions of your work.

If you plan to print the image, then you have to be aware of the output resolution of your printer. There's a point of diminishing returns here. You might capture or scan an image at extremely high resolution and wind up with an enormous file, but for most inkjet printers, anything higher than 300 dpi (usually less) is wasted.

A higher resolution printer will get best results from higher resolution images. If you are working with a service bureau or published journal the easiest thing to do is ask them what resolution (and image format) is best. If you are printing on a standard laser printer in Life Sciences, 120 to 150 dpi is about right. If you have your own color inkjet, it’s best to experiment but it will probably be in the same range, though the newer photoquaility printers may produce excellent results at 200-300 dpi. As I said, experiment with your printer..

Another another thing:

Using the zoom tools in Photoshop changes only the display of the image on your monitor. It doesn't affect the file size or print dimensions. To change either of those, you have to go to Image>Image Size and make your adjustments.

Digital Cameras ands Megapixels

When determining how good a camera to buy, or how to set the one you have, you have to know what photo size you'll want to print. Here's a handy guide from http://graphicssoft.about.com/cs/digitalimaging/f/pixelsprint.htm

5 MP = 2592 x 1944 pixels
High Quality: 10 x 13 inches
Acceptable Quality: 13 x 19 inches

4 MP = 2272 x 1704 pixels
High Quality: 9 x 12 inches
Acceptable Quality: 12 x 16 inches

3 MP = 2048 x 1536 pixels
High Quality: 8 x 10 inches
Acceptable Quality: 10 x 13 inches

2 MP = 1600 x 1200 pixels
High Quality: 4 x 6 inches, 5 x 7 inches
Acceptable Quality: 8 x 10 inches

So for most of us, all we ever will need is around 3 megapixels. How many 8 by 10 prints do we want anyway? Still, the higher ranges come in handy if you want to crop the photo, or for the occasional special photo you might want to send to an online printing service to be printed in a large format.

Scanning 35 mm Slides:

Here's another area where acquiring the highest possible resolution may not be worth the effort. Here you're restricted by the resolution in the original film grain at the time the photo was made - some films are more detailed than others.