This is a security feature created to make your email account more secure.
With special image technology, spammers are able to discover whether your email ID is valid. When you open messages having these embedded codes, the sender learns that your email ID 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 the original.