This is a security feature created to make Rediffmail user's email accounts more secure. This is because using special image technology spammers try to discover whether your email address is valid. When you open messages having these embedded codes, the sender learns that your email address is active and then continues to send you unsolicited emails.
In Rediffmail, we prevent these images from opening until you verify their source. The images are not deleted; they are simply masked with grey colour until you choose to see them.