I've just been doing a bit of work on an icon for Fez object cloning. I picked an icon which shows two pieces of paper next to each other but on the white background they were hard to see. I decided to add a grey tint the icon so it would be more visible.

As a bit of an open source junky, I thought I'd have a go at Paint.net as a lightweight image editor. I opened the icon, created a new layer, selected the two bits of paper with the wand tool, pasted grey onto the top layer and then changed the transparency of the layer until I could see the paper icon underneath. Paint.net was pretty easy to use and it's free so I recommend if you are just doing simple tasks and don't want to pay a license for full-on image editing software.