(CNN)  -- Liverpool striker Ryan Babel has been fined £10,000 ($15,890) by the English Football Association (FA) for posting doctored photos of referee Howard Webb on his Twitter blog. Babel was found guilty of improper conduct Monday by an FA regulatory commission and warned about his future conduct. He is the first Premier League footballer to be charged and fined for "inappropriate" tweeting. The Dutch international used his Twitter account to criticize Webb, who took charge of Liverpool's FA Cup third round match against Manchester United. World Cup final referee Webb awarded United a disputed penalty and then sent off Liverpool captain Steven Gerrard as they slipped to a 1-0 defeat in Kenny Dalglish's first match at the Anfield helm.
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NEW: Manchester United's Rafael charged with improper conduct
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Ryan Babel is the first footballer to be charged by FA over a Twitter posting
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Babel used a mock-up picture of referee Howard Webb and made derogatory comments
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The Dutch international could face a ban if found guilty of charges
Babel had run into trouble on @placeholder before, being forced to apologize to former Liverpool manager Rafael Benitez for comments he published after being dropped.