Adobe Photoshop : File size and resolution

For example, if you scanned a low-resolution image at 3" x 5" and later wanted to increase it's size to fill a larger space -- say four times the size of the original -- you could do so, but the resulting image would have one-quarter the resolution of the original. Why does this matter? Since the pixels would be stretched to cover much more screen space, the image would end up looking very "rough" (a phenomenon known as "pixelation"). The images below illustrate this idea:

JPEG image opened in Photoshop.

The same image, four times larger and with only 1/4 the resolution of the original.

See the difference?

So when inserting images from Photoshop into another application, don't think you can just grab an image and stretch it to an infinitely large size. You'll lose resolution when you do, and the image will look bad. The same goes for resizing in Photoshop: Photoshop can't add pixels to an image to increase its resolution.