How do I make partially transparent layers on a web page?

I have a photography site and I want to give the user the chance to see what a picture would look like with different frames and mattes. I think I can write the javascript if I can figure out how to make the inside of the image of the frame transparent, and how to lay that over the original picture. Can anyone offer advice as to how to go about this? I'm also open to easier methods of achieving this same effect. I'm an ok html programmer, but fairly new to any other languages. How do I make partially transparent layers on a web page?
You can use the CSS attribute ';opacity'; to do this. Here's some info on it: http://www.w3schools.com/Css/css_image_t鈥?/a>





I would suggest you also take a look at MooTools (http://mootools.net/). If you're not familiar with MooTools, it's a free open-source javascript framework that will let you create some really nice effects--including transparency--that would make a really great way to preview various frames and mattes on your photography site. I found the tutorials and documentation on Mootools site to be far inferior to the tutorials that you can find on this guy's site: (http://www.consideropen.com/blog/2008/08鈥?/a> so if you get frustrated with the Mootools site, take a look there.

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