You can change its size/opacity dynamically, and even save the result into a separate file. HTML5 canvas provides a simple method to get a URL (in data: scheme) that holds the drawn image. (Yay! I don't have to open an image editor anymore just to scale the size of my icon :)
Of course, you need a browser that has HTML5 support, e.g. Safari, Chrome, Firefox, Opera, or something other than Internet Explorer. There's an unfinished yet beautiful book about HTML5, and a nice blog post introducing new features of HTML5.