Today's Image of Mars is of the Face on Mars, one of the most famous pareidolias on Mars.
In 1976 the Viking I Orbiter was taking photos of possible landing sites for Viking II when it took the below shot of what looked like a human face. Scientists quickly realized this 'face' for what it was, a mesa with irregular shadows that at the right angle made it look like the face of an ancient Egyptian pharaoh.
The Face on Mars soon captured the public interest, but what many people don't realize is that NASA wanted that to happen. They initially published the photo with a caption "huge rock formation ... which resembles a human head ... formed by shadows giving the illusion of eyes, nose and mouth." They the created a public relations campaign around The Face on Mars. According to NASA "the authors [of the initial caption] reasoned it would be a good way to engage the public and attract attention to Mars." They were successful - perhaps too successful.
The public became fascinated by The Face on Mars, with some theorizing it was an alien artifact being covered up by NASA. NASA felt it was important to dispatch these claims, so they imaged it with the Mars Global Surveyor in 1998 and 2001. [See those images in this NASA article]
Regardless of evidence, there will always be people that wish to believe in conspiracy, so NASA hasn't convinced them all, despite having a high resolution image of The Face. NASA has even pointed out they would have no reason to deny a civilization on Mars because their budget would surely increase if one was discovered. Alas, the Face on Mars is just one of many mesas in the Cydonia region of Mars.
Clicking on this image will take you to the 2001 article about the Face on Mars and the various images taken of it.