As far as I can work out, there is no native facility for importing your own icons in to the extension, but as I am not WA people, perhaps a more authoritative WA response will confirm or challenge that view, and clear it up!
I agree that for many, a YouTube button would be a major benefit to this extension, and I expect that the more people who request the feature via the feedback forms or wishlist areas, the more likely we are to see its inclusion in a new release.
With MySpace, I am less convinced, given the internal troubles that MySpace (which News Corp is desperately trying to off-load) are experiencing. Also, their recent maneuverings in order to link MySpace pages to FaceBook accounts are (according to gigaom) "a final sign of how completely it has surrendered to its former foe".
While MySpace still has the largest share of any social media networks in the US with 72 million monthly uniques larely, Im guessing, because of its music and media capabilities, its international appeal is increasingly outstripped by Facebook. My guess is that the company will be sold off for peanuts, and disappear altogether as FaceBook's capabilities and UI grow. More graphical info here.
But YouTube, yes definitely.