Maybe if you just do a large scale (300x300px), then convert the logo to 4 colors (the 3 shades of green + the transparent bg).
I use my gravatar photo as a favicon:
http://kenCode.de I'd like to see MetaExchange get tons of brand recognition like shapeshift, especially since i am starting to use ME now more often than shapeshift
The Pixlr editor lets you design right down to a 16x16 icon scale too..