What should a girl do after registering her email in "Youth Chat for Boys and Girls" (شات شباب وبنات), then regretting the disgraceful messages she receives after posting a picture of an American girl?

The internet is a useful tool if used properly, and harmful if misused. You made a mistake by putting your email address and a girl's picture in the chat, because that tempts those with diseased hearts. What is incumbent upon you is repentance, and the best way to rectify the situation is to delete this email and create another address if needed.