Digital Images and Photographs

A digital image is a picture stored as numerical data so that computers can display, edit and transmit it. Most photographs are raster, or bitmap, images: they are divided into a grid of tiny picture elements called pixels, each holding a colour value. The number of pixels across the width and height defines the image resolution, and a higher resolution records more detail at the cost of a larger file. Wallpapers, screensavers and stock photographs are all examples of raster images at various sizes and resolutions.

Different file formats suit different needs. JPEG uses lossy compression that discards some detail to make files small, which works well for photographs of nature, landscapes and people. PNG and GIF use lossless compression and support transparency, which suits graphics, logos and screenshots. Photographers also work with high-quality formats that preserve more information for editing. Choosing a format is a trade-off between file size, image quality and features such as transparency or animation.

Because the web contains an enormous number of pictures, image search engines help people find them. These tools index images by the text near them, file names, captions and descriptive tags, and many can also analyse the visual content itself to group similar pictures or find where an image appears online. Licensing matters when reusing photographs: some are released for free use, some require attribution, and others are fully copyrighted, so the terms attached to each image should be checked before it is copied or published.

Frequently asked questions

What is the difference between JPEG and PNG?

JPEG compresses photographs into small files by discarding some detail, while PNG keeps every pixel and supports transparency, making it better for graphics and screenshots.

What does image resolution mean?

Resolution is the number of pixels in an image, usually given as width by height. More pixels capture finer detail but produce a larger file.

How do image search engines find pictures?

They index surrounding text, file names, captions and tags, and many also analyse the visual content to match or group similar images.