I would like to know if it is all right to masturbate to girls' pictures where they have clothes on.
"Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts" (Romans 13:13-14).
The problem is lust. Lust is strongly wanting to do what is immoral, which would include the fornication. So while you are looking at pictures of girls clothed, what you are imagining as you're masturbating is having sex with them, which would be sinful since you are not married to them. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).
Another problem is that you are teaching yourself to see girls as objects for sex instead of people. Think of it this way, how would you feel if you knew some homosexual was masturbating while looking at your picture? "Therefore, whatever you want men to do to you, do also to them, for this is the Law and the Prophets" (Matthew 7:12).