About digital images
Digital images are photographs, artwork, diagrams, icons or any pictorial material which can be viewed on a computer.
Images can convey meaning and clarify concepts.
Resolution
To use an image digitially it must be stored as a series of pixels. The number of pixels per inch or dots per inch (dpi) of an image refers to its resolution. An image with a higher resolution such as 300 dpi requires more storage space and contains finer detail.
Images for web sites should be a relatively low resolution such as 72 dpi, due to the limitations of the computer screen and the need to conserve file size due to data limitations.
Images which should be printed need a higher resolution such as 300dpi or more, as printers are capable of finer detail. Printing an image at too low a resolution will result in noticeable lack of detail sometimes called 'pixellation'.
File formats
Images for the web need to be in one of the standard web formats: JPEG, GIF or PNG. Images saved in other formats, such as BMP, will not be viewable on some machines.
The JPEG format is designed for photos and artwork containing photos.
The GIF or PNG formats are designed for simple non-photo images such as buttons, logos and graphs. PNG and GIF serve the same purpose, with PNG being a newer format.
Sometimes you need to experiment with both formats to see which looks nicer and which creates a smaller file size (thus more efficiently storing the image).
Terminology
Some of the organisations working in the area of digital
images provide glossaries defining commonly-used terms:
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