Pixel-based images

Pixel-based images, also referred to as Raster graphics or Bitmaps a made up of small squares or dots known as pixels. A pixel (pixel-element) is the smallest editable element in a pixel-based image. A pixel-based image may be stored as a number of file formats such as a jpeg, png, gif, tiff or psd. Unlike Vector based imagery, pixel-based imagery is resolution dependent and can not be scaled up without loosing quality. Resolution is typically measured by the number of pixels per inch (ppi). The higher the number of pixels per inch the higher the image quality will be and the larger the file size will become. Each pixel in an image has a position on the document and an RGB value. By altering the RGB values and positions of various pixels in an image using a pixel editing program such as Adobe Photoshop, we can retouch and alter an image in a vast variety of ways. All digital photographs are pixel-based as are the majority of of images within a web page.

This video takes an imaginary journey into the land of the pixel:

Uploaded by onemoreprod. – Watch original web videos.

Understanding Resolution

The term resolution refers to to the amount of information in an image.
Devices such as digital cameras, printers, scanners & monitors all have a particular resolution.
Image resolution is measured by the amount of Pixels Per linear Inch (PPI) or Dots Per Inch (DPI) if referring to print resolution.
The more pixels per inch the higher the image quality (and the larger the file size will be) .
High quality print resolution is typically 300ppi. 300ppi is roughly the limit of what the human eye can distinguish.
Monitor resolution varies greatly depending on the device (from a 19ppi TV to a 340ppi mobile phone). Find the PPI of your monitor in this Wiki entry.
Unlike when printing, an image will be displayed on a monitor pixel to pixel so will change size depending on the devise.

Here’s a pretty simple breakdown of resolution.

Here’s a well written and relatively easy to grasp explanation.