The Face In The Background
He finds the picture on Cordy's desk by accident, looking for some fuiles on their latest case. A corner of the polaroid peeks from a corner of Cordelia's dayplanner and curiosity makes Wesley pull it out and examine it closely. Taken during a rare moment of leisure, it shows a nice close up of Sunnydale's former May Queen, but something in the background of the image catches Wesley's attention.
Oh, he remembers that day: He'd been sitting next to Angel, reading ancient prophecies, while Cordelia did a little investigation on the internet. With the young female almost shielded by the door and at a safe distance, words had been whispered, hands had hovered over cloth and barely exposed flesh. Even promises of cool embraces under the spray of a long shower had been promised (and then made true). The next day things had gone back to normal. Maybe it would happen again, maybe not, their fascinating little secret.
Maybe not so secret, judging by the picture.
Cordelia could had easily miss it at first, but she has such a keen eye for detail. By now she must have noticed the background of the picture,Wesley's reflection is a nearby mirror, captured forever by the magic of the polaroid. And of course, she's smart enough to know Wes is not kissing the cool, velvety rough lips of thin air.