Postcards come in many sizes and shapes and they are very versatile in their use. They can be used as handouts, gift certificates and direct response mailers. Direct mail postcards are the workhorse of direct response mailings. Direct response marketers have used postcards for decades because it works. It is still one of the most effective and low cost means of advertising.
You have limited space and thus, you must use it wisely. Pick one dominant image and not because it's pretty but because it's effective. Identify and engage your audience with something that is eye catching, relevant to the headline and most importantly, relevant to your reader in some way.