Paint.Net which is free and easy enough to use. Plenty of
tutorials on their website too.
http://www.getpaint.net/
Tutorial on fading/blending
http://paintdotnet.forumer.com/viewtopic.php?f=15&t=2756&sid=f3baa192abb4ccb9a2571c80a5d7dede
I can't turn the link into an actual link because the HTML
editor will screw up the ampersand sign.