I'd use ImageMagick, or the pipeline: djpeg | pnmscale | cjpeg. I'd certainly not use the HTML method: that's the worst of two worlds: you download the full image, but only display a distorted image.

If not having ImageMagick installed is a problem, just bundle whatever you need with your product. There are many image modifying programs available, and no doubt there's one with a license that allows you to distribute it.

Abigail