Mac OS X Hints has an article on adding the standard feed icon to Safari. of course, I already created one and put it on my website. I also sent in my own hint linking to my icon; my hint was not accepted.
the Mac OS X Hints post has several mentions of different Safari feed icons in the comments, including me mentioning my own. one problem with the hint's instructions is that the generated TIFF file will not have an alpha channel, leading to the ugly black corners that you see in the screenshot. AFAIK, none of the other renditions have this. another problem is that it doesn't affect the 'Hide' button - when you go to a feed page, the button will change to say 'RSS' again.
you should try all the versions, as everybody seems to have a different taste for how the 'Hide' button should work. if you can't find one you agree with, feel free to make your own interpretation.