how to create a .png semi-transparent image for background

I want to make the background of my tables semi-transparent, but putting an alpha filter on them also makes any text or images with in transparent. To avoid this I want to set a url image as my background. I've read that png images will hold their transparency settings when used like this, but I don't know how to go about making the png. I tried saving a microsoft picture It image as a png and then uploading it to photobucket to get a url, but it doesn't seem to work. Any suggestions are greatly appreciated, thanks!
Laura

If you use photoshop or fireworks you can create a picture with transparencies. Now it is true that you can use transparent images with your backgrounds. You can see some of the work I have done at the most current site, www.beach-cruise.com. Now note this, IE does not support PNG transparencies directly, but I have gotten it working by doing some extra coding based on some work from another before I.

If you notice the site has transparent PNGs in the header of the site and scroll across the top as it has been re-sized. Also all signatures and other transparent PNG's are all taken care of by this code. If you would like I can go into greater detail, but I am going to be writing up an article on this when I post my next web template I design.

You can read more about this at http://koivi.com/ie-png-transparency/

You can "fake" transparency by makeing the background of the image the same colour as your webpage, of course if you plan to use the image on more place and each place has a diffrent colour this isn't realy an option.