Correct. IE6 does not support png alpha transparency.
I would simply convert it back to a gif and add a suitable matte. Another solution would be to make a gif version and add the following to tweaks.css:
* html .online {
background-image: url("{T_IMAGESET_LANG_PATH)/icon_user_online.gif");
}
Or if you want to fix all the png images at once, there's some javascript solutions out there that will also work.