Resolution is a concept that continues to baffle even graphic artists. In the context of editing photos, resolution is a measurement of the output quality of an image. The most common units to measure resolution include: PPI (pixels per inch), DPI (dots per inch), LPI (lines per inch), and SPI (samples per inch). For our purposes, we will focus on DPI and PPI because that is what you will be dealing with most often when printing photographs.
MP (Megapixels) simply means "one million pixels" and is used when describing digital camera capability. Some digital cameras now boast photo abilities over 5 megapixels.
PPI or "pixels per inch" is the term you will see most often when selecting a resolution for your images in photo editing software. Pixel is an abbreviation for "picture element." Millions of pixels make up the image of paper and text that you are viewing on your screen right now.
So how do you best choose a pixel resolution for your photos? First, you need to consider where you will use your pictures (in print or online) and how large they will appear. For best results, a good number to remember when printing an image is 300 pixels/inch at actual size. In other words, when you resize an image with photo editing software, set the dimensions of the image to the print size first (e.g. 5 x7) and then set the resolution to 300 pixels/inch. The lower the pixels per inch, the blurrier the image will appear on paper. Depending on your printer, you could get acceptable results from 200-300 pixels/inch. In contrast, an image from the Internet is typically only 72 ppi. When you try to enlarge a picture with such a low resolution further, you are asking the software application to make up for pixels that don't exist; the image will get blurrier. So, what pixel size do you need for great printing results with the least strain on your computer memory resources? Below is a quick reference chart.
5 MP = 2592 x 1944 pixels
Great Quality: 10 x 13 inches
Good Quality: 13 x 19 inches
4 MP = 2272 x 1704 pixels
Great Quality: 9 x 12 inches
Good Quality: 12 x 16 inches
3 MP = 2048 x 1536 pixels
Great Quality: 8 x 10 inches
Good Quality: 10 x 13 inches
2 MP = 1600 x 1200 pixels
Great Quality: 4 x 6 inches, 5 x 7 inches
Good Quality: 8 x 10 inches
As you can see, 2 MP is typically sufficient for printing great quality 5 x 7 photos. You may want to save your photos with a higher MP if you want to be able to crop sections out of a photo and still have quality composition.
Consider one thing when selecting resolution—you can always scale down your resolution but you cannot increase the resolution of an image. So, in this case, bigger is better if you can accommodate the extra file sizes on your computer and printer.
DPI specifically refers to how many dots of ink will print per inch. The higher the number, the sharper the image will be. Most ink jet printers today are capable of printing 1200 to 4800 dpi; this means great results for images with 200-300 ppi.
Online or Screen considerations
The magic number for screen formats—TV, computer monitor, PDA or cell phone—is 72 ppi. The size in pixels (not in inches) determines how big on the screen you want your image to appear. If your monitor is set to 1024 x 768, then an image at that size set to 72 ppi will fill the screen. Since image size (dimensions) determines file size (the number of bytes, k) you shouldn't make the image any larger than necessary for sharing photos through email or online.