image resolution.

Describes the detail an image holds. High resolution images should be a minimum of 300 dpi (dots per inch). If the image needs to be enlarged the dpi will decrease.

What this means to you?
If you provide us with a 4" x 6" image at 72 dpi and want it to be a 2' x 3' poster your image will be very pixelated. In other words by keeping the number of pixels constant, and we enlarge the image, then the size of each dot grows and the image becomes grainy, as the resolution of the eye enables it to pick out individual pixels.

The same does not hold true for Vector graphics as they store the lines, shapes and colors that make up an image as mathematical formula. Printing the image gives a sharper, higher resolution output than other images. Though Vector graphics are usually made in Adobe Illustrator this does not mean you can place a jpg image in illustrator and expect it to print perfectly as it is still a jpg image.