Many image formats, including JPG, include a "dots per inch" (dpi) or "pixels per inch" (ppi) setting as part of the file. This setting is a single number stored as part of the JPG file, and is used my most programs in determining the scale at which to print the image. This setting has no effect on screen image size in nearly all web browsers. (Are there any that do?) The number is ignored by most software in displaying images on screen, but is used frequently in determining print size.
Here are two pictures. Both are 350x262 pixels, but the ppi settings are different:
They probably appear the same size in your browsers. Most browsers display each image pixel as one screen pixel and pay no attention to the pixels per inch value.
However, printers may render the first more than four times larger than the first. Try downloading each image into your graphics program and printing.
Here is what the fuss is about. Many graphics programs and browsers refer to this setting discussed above as dots per inch. Some people prefer the terminology "pixels per inch" when discussing digital images, to avoid confusion when talking about how images are rendered on printers. Most printers can render many more dots per inch than the pixels per inch we call on them to print. This is good, because most printers, unlike monitors, can render very few colors, and so must average over many dots to render a particular color with reasonable accuracy. A 1440x1440 dpi printer can devote a 20x20 cell of dots to render a single pixel of a 72 dpi image. If the pixel is a light pink, a four-color printer will have a few of these 400 dots (20x20) red, while the remainder are white. Thus, someone speaking precisely would say that the image is 72 pixels per inch rendered on a 1440 dots per inch printer. (Most people and programs do not bother with this distinction.)
Some printers have a bit more control than the above would indicate. Some printers have variable dot size, while "photo" printers add two lighter toned ink colors to the standard cyan, yellow, magenta, and black. In any case, you need a lot of printer dots to accurately render the color of an image pixel, so you always need a printer with many more dots per inch than the pixels per inch of your image.
Miscellaneous tips and info