If those people do not want their work to be used by anyone then they should not be posting it on the internet.
Hell I used screenshots of the original HL to help texture my players and weapons for a mod that is essentially a remake of the original but in 2D format.
Yes make skins your self pixel by pixel if they are original but if you are remaking an existing piece or art then yea its free and fair usage as far as im concerned. Its the same as remixing songs in that you take something that already exists and make something new out of it.
And Yates calm the fuck down its just a game and if you don't like it then don't rate the skins just ignore them. Dont waste your precious time flaming on other peoples work just because you don't agree with it. As after all the whole fucking world does not revolve around you and your opinions. >>