Create the "Windows" images you want to use on separate layers, then change the Opacity value for each layer until you get the look you are after.
Use the Save to Web dialogue and save your final images as JPEG, you can alter the quality settings to get the best comprimise between image quality and image size.
Then save your orignal file as a PSD, so that you have a layered file to work on in the future, should you need to.