File Size refers to the number of bytes (storage space) required to save a file to a disk.

Image Size refers to the dimensions of an image.  An image's screen size refers to how large it will appear on a computer display, usually measured in pixels; whereas its print size indicates how large the image will be when printed onto a page, usually measured in inches.

Image Resolution is a measurement of the output quality of an image.  Another way to think of it is as the density of an image. The terminology varies according to the intended output device. Resolution is usually referred to in terms of pixels for screen display, and in terms of dots for printing.

PPI stands for pixels per inch.  PPI refers to an image's screen resolution, or display resolution.  A digital image is composed of samples that your screen displays in pixels.

DPI stands for dots per inch.  DPI is a measurement of printer resolution that defines how many dots of ink are placed on the page when the image is printed.  DPI does not correspond directly with PPI because a printer may put down several dots to reproduce one pixel. This is because printers use a limited number of colored inks to reproduce an image consisting of millions of colors. The higher a printer's DPI, the smoother your printed image will appear, provided you have a suitable amount of image resolution (PPI).

The term DPI is often used interchangably with PPI, causing a lot of confusion; however, PPI is screen resolution, and DPI is printer resolution.  How can you remember this? Monitors display pixels, and printers produce dots.  

Although PPI is a screen or display resolution, it influences the size an image will print. An image with dimensions of 1600 by 1200 pixels and a resolution of 300 PPI will print at a size of 5.3 by 4 inches. Or it could be printed at a resolution of 180 PPI for a printed size of 8.89 by 6.67 inches. The higher the PPI value, the better quality print you will get--but only up to a point. 300 PPI is generally considered the point of diminishing returns when it comes to ink jet printing of digital photos. Today's photo-quality ink jet printers have DPI resolution in the thousands (1200 to 4800 dpi). They will give you acceptable quality photo prints of images with 140-200 PPI resolution, and high quality prints of images with 200-300 PPI resolution.

A popular myth is that it's necessary to scan an image at the same resolution that you will be using to print. In the case of color and grayscale images, this only results in excessive file sizes.

Often images are referred to as high resolution (hi-res) or low resolution (low-res). High resolution would be an image intended for print, generally having 300 samples per inch or more. Low resolution refers to images only intended for screen display, generally having 100 pixels per inch or less.

Scanner and digital camera manufacturers often refer to two different types of resolution when listing product specs: optical resolution and interpolated resolution. In reference to scanner and digital camera specifications, the interpolated resolution is often listed along with the device's optical resolution. The optical resolution is the true measurement of image resolution that the output device can capture. Interpolated, or digital, resolution is acquired artificially via a process where the software adds new pixels to an image based on the color values of the surrounding pixels. Interpolation is used when an image is upsampled to increase its resolution. However, resampling through interpolation is not ideal and often results in a blurry image. In this situation, the optical resolution is the true measurement of what can be captured, and you should disregard the interpolated resolution as a purchasing factor.
 

Digital photos and scans are all bitmap graphic types,
no matter what format the image is saved to. That means
they are made up of a grid of individual, tiny pixels with each pixel representing a single color in the image. If you zoom in on one of these images (see example) in your imaging software you'll be able to see the individual squares of color. (See illustration at right)

What this means in relation to printing is that these types of images are resolution-dependent. In other words, the quality of the print and the size of the printed image is limited by the number of pixels in the image. You can't increase one value without effectively decreasing the other.

Because monitor resolution is so much lower than printers, low resolution images that look fine on-screen often print poorly. The main problem with printing Web graphics is that as they're transferred to the print environment, PPI become DPI. Each pixel is seen as a one dot. And although 72 PPI is fine for looking at graphics on a screen, it's a very low DPI resolution for printing.