Web browsers can read many types of image files. But to make your page download quickly, there are two basic formats which produce small files: JPEG and GIF.

JPEG files are best for photographs. Usually in photos there are smooth gradients of shade, and the structure of JPEG files can maintain that smoothness even if the file is highly compressed.

GIF files are best for diagrams or text, in which there are solid blocks of color and little or no transitional shading. The reasons for this are beyond the scope of this tutorial.

General rules for web images:
Crop as closely as possible around the subject of the image.
Keep the resolution at 72 pixels per inch.
Shrink the image to the smallest possible size at which the subject is easily recognizable.
Compress the colors and file-size up until the image starts looking rough or fuzzy.

Ideally, images within webpages should be 50 kb max; preferably half that.

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