Ed, if you don't want to download anything you could try
this[
^] free web-based image editor. I've just tested it and converted the white background of my sample
PNG image to transparent. I don't know if it is the only way but I've filled the background with transparent color using the 'Paint Bucket' tool and the job was done.
If the download is not a problem, but you don't want to install the program then you can use the
CxImage demo executable[
^]. It does not require installation. And setting the transparency is very easy - just right click on your image (loaded in
demo.exe), choose
Set Transparency and select your transparent color. :)