On my small website I have two lines in my head tag:
<link rel="icon" href="http://website.com/favicon.png
<link rel="icon" href="http://website.com/favicon.ico
That way I have a normal favicon and a speed dial image. I don't remember the recommended size for the .png, but mine is 256x160.
I do sometimes see the image gone from my speed dial, but because I normally use Opera Beta I can't really complain 