Banner ads are usually designed by someone other than a webmaster. Even if they are designed by a webmaster and are destined for other sites, the file size is of little consequence to the designer.

Now, if you're a webmaster receiving a banner ad for your client's site, then file size is extremely important. Unless the banner ad is the focal point of the page, I would not want one with a file size over 10kb. That is just a guideline. Regardless of whether it's broadband or dial-up, with all else being equal, the faster page wins in ecommerce.

Some sites exchange banners with other sites as a form of reciprocal linking. If you get several large banners, the page loading becomes excruciatingly long regardless of connection speed. In those cases, I would highly recommend compressing the image as much as possible while still retaining some visual quality. Also, I would only use one frame of an animated GIF for such a banner.

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DrTandem's San Diego Web Page Design, drtandem.com