Thanks FL310.
I've just been playing with a Program I've found on the web called NeoPaint for Windows. It does the trick for creating icons very well. It'll cost $60US if you use it after the 30 day evaluation period.
Go here:
http://www.neosoftware.com/npw.html
It'll take a while to get the hang of sizing the final product so that it looks right on the Desktop. The default Windows Icon seems to be a ½ inch square and on my screen that comes to about 40 pixels each way. The PPRuNe icon is rectangular but it fills that square and is therefore distorted. But at least it's now different from the others!
So it's possible, but more work required to achieve perfection......