These icons are in PNG format and are transparent, so they are suitable on any background. The "banners" however have a light border, they do look best on light to medium backgrounds but dark is not out of the question!
Free Twitter Icons & Graphics
Sorry, this file is no longer available :(
Update: I have created Twitter Sweet, a new set of free Twitter icons you may be interested in since this set is unfortunately no longer available...
Subtle Grey: Free Social Network IconsTwitter Sweet - 26 Free Twitter IconsLighter Grey: Free Social Network IconsTwitter Status Display with Blogger How-To