Graphics

Graphic files tend to be rather large, even with the advanced compression techniques used today. The Page Size section provides caution and guidance for limiting the total combined sizes of page text and graphic files.

If a large graphic needs to be incorporated into the site, it is best to place a scaled down version (a thumbnail picture) on the page. Then a link can be provided to access the full size version, and the file size should be indicated so the user can decide whether or not to wait for the image to download.

The way a page and its graphics are downloaded, is the file containing the text content of the page and the code for the layout of the page, is sent to the browser. After the text file is downloaded, the browser makes calls to the server (web site host computer) to retrieve the graphic files. Previously, the browser waited until the image file was received before it displayed the page on the monitor. The reason for delaying the display was because the browser had to receive information concerning the physical size of the images to correctly display the page.

Most users today now have browsers that are capable of interpreting the codes in the text file concerning the physical sizes of images. The graphic size code (the image tag's width and height attributes) allows browsers the start laying out and drawing the page before any image files are receive. Proper site design must ensure that this code is included for images.

The image tag's ALT attribute is something many designers forget to add. This attribute provides alternative text for the user who is not or cannot view the images. Even when the image is purely decorative the ALT attribute should be used to suppress the display of an image marker on text only browsers. In these cases a blank is used for the attribute's value (ALT=" ").

Image formats most commonly recognized by graphical web browsers are GIF and JPEG. Newer formats, such as PNG are not widely supported as of yet. Formats commonly used on computers, such as TIF, BMP, PICT, and PCX tend to have fairly large file sizes (compared to GIFs and JPEGs) and would require significant download times if used. The GIF and JPEG formats (usually an extension of jpg) are compressed files. The GIF format works best for line drawing and images having solid colors without blending. The JPEG format works best for photographs.

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