Image Resolution

Resolution can be difficult to understand. 
The following is a basic look at resolution that may answer some of your questions.

Dots Per Inch (or DPI)

DPI, or dots per inch, refer to printer resolution. These are the actual number of dots that can be printed per inch on paper.  300 to 600 DPI is standard for most printers, keeping in mind that the higher the DPI,  the clearer the printed picture.

Pixels Per Inch (or PPI)

Your monitor displays everything in PPI or "Pixels Per Inch". Most monitors display graphics at 72 pixels per inch, so if your graphic is 72 pixels by 72 pixels, it's actually one inch by one inch.

When building a web site you should keep in mind that about 50% of viewers will be using a monitor resolution of 1024 x 768, and about 50% will be using 800 x 600, the two most widely used resolutions.

You should view your pages at different resolutions to get a good idea of what others are seeing. A resolution of 800 x 600 is easier to see because the content is larger than 1024 x 768.

Scanning

A digital image is composed of pixels, which are the small colored square dots. Resolution is the number of pixels wide by the number of pixels high. A picture that is 1152 pixels wide and 768 pixels high has a resolution of 1152 x 768 pixels. Scanning should should be done only at the highest resolution needed. 

72 DPI is perfect for web use, while 300 DPI or higher is best saved for print use. Scanning at a high DPI than you need will result in a much larger file than you need, but will also eliminate the need for re-sampling.

Estimating File Size

Inches scanned x resolution used = Image size in pixels.

The image size you want depends how the image is to be used. A higher resolution results in a larger image that requires computer memory to open and extra hard drive to store. In argument of size, the higher resolution will also result in a much better printed result.

(6 inches x 110 dpi) x (4 inches x 110 dpi) = 660 x 440 pixels will nearly fill a monitor screen whose resolution is set at 640 x 480.

(6 inches x 140 dpi) x (4 inches x 140 dpi) = 840 x 560 pixels will nearly fill a monitor screen whose resolution is set at 800 x 600.

(6 inches x 180 dpi) x (4 inches x 180 dpi) = 1080 x 720 pixels will nearly fill a monitor screen whose resolution is set at 1024 x 768.

Re-Sizing

If you need to resize an image, it's a good idea to remember this: You cannot ADD pixels to any photo. What you end up with is a "pixellated" image, meaning you've made the existing pixels larger than they were causing the image to appear blurred.

Re-Sampling

Re-sampling doesn't change image resolution, but instead changes the pixel dimensions and can result in poor image quality.