I don't know if this is any help, but I have been using Macromedia Fireworks, (I believe it is the older version of Adobe Fireworks), and it has the easiest transparency slider to use. Obviously its quite expensive so its not a great option. However if you do have it already, then it works a charm.
I looked through a couple of other forums for free options and this link came up,
http://www.snapfiles.com/get/photofiltre.htm
Not sure if its any good but you could give it a try,
Holly