Pixel dimensions and image resolution
The number of pixels along the height and width of a bitmap image is called the pixel dimensions of an image.
The resolution of an image is determined by the number of pixels per inch (ppi) printed on a page. In Photoshop, you can change the resolution of an image. In ImageReady, the resolution of images is always 72 ppi, to optimize the images for online media.
When printed, an image with a high resolution contains more, and therefore smaller, pixels than an image with a low resolution. Higher-resolution images can reproduce more detail and subtler color transitions than lower-resolution images because of the density of the pixels in the images. High-quality images often look good at any print size.
You can't improve a lower-quality image by printing it at a high resolution. Changing the print resolution of an image simply makes each pixel larger, which results in pixelation--output with large, coarse-looking pixels. Increasing the print resolution of an image doesn't add any pixel information to the image. You can make a low-resolution image look its best by picking a print size that makes the most of the pixels it has. For more information on print resolution, see Printer resolution.
It's important to note that video files are displayed only at 72 ppi. Even if an image has a higher resolution than 72 ppi, when it's displayed in a video editing application, the quality may not look very good.
