The short form:
#1, First thing, you should ignore resolution.
On its own, it ismeaningless. You have to combine it with a physical size.
For example, take a file that is 2000x1000 pixels. What is the resolution?
The answer is 72 dpi, or 100 dpi, or 200 dpi, or 300 dpi, or whateveryou want it to be. The DPI by itself means nothing.
That 2000x1000 pixel image will be:
at 72 dpi = 28 x 14 inches
at 200 dpi = 10 x 5 inches
at 300 dpi = 6.5 x 33 inches
That's all from the same file with no changes to it.
Typically, a 6 megapixel image will be something like 3072x2048pixels. If you set that to 180 dpi the size in inches will be 17.067x 11.378 inches. If you set it to 300 dpi it will be 10.24 x 6.827 inches.
As there are a number of 8 (and greater) megapixel cameras out now,here are the numbers for an 8mp image:
19.467 x 12.978 inches @ 180 pixels per inch
11.68 x 7.787 inches @ 300 pixels per inch
Some camera files claim to be 72 dpi. Others claim to be 180 dpi.Others might claim some other resolution. The amount of pixels is the important number. You can change the resolution at any time without affecting the file size/quality. Go to Image > Image Size and uncheck the RESAMPLE box. Then you can type in whatever resolution you want. The document size will change, but the pixel dimensions will not.
The Longer Version (click here).