I think showing refers to a lot more then just describing the picture. Showing can be showing a characters emotions, showing tension, showing actions. Let me try an example and see if it works:
Telling wrote:
The gunman walked into her cell. Anne looked up. 
"It's time." He said gruffly.
Anne rose and followed him out the cell door and down a long hallway. This was the price she had to pay for defending the people she loved.
As they reached the designated room, she was told to sit in a steel chair facing the wall.
The gunman stood in front of her and raised the gun. This was the end. 
Not much colour there. Maybe showing is like technicolour tv, as opposed to black and white films.
Showing wrote:
The burly gunman entered the cell. His presence seemed to fill the room like a black cloud. Anne looked up, her heart hammering in her throat, threatening to choke her.
"It's time," He said gruffly, with the barest hint of compassion.
Anne rose on shaky legs. The hallway seemed to stretch endlessly in front of her as she followed the gunman. The longest walk of her life, and yet the shortest at the same time. Her vision clouded with tears, but she knew it was worth it. It was a small price to pay for the people she loved.
As they reached the designated room of peeling yellow paint, the gunman gestured for her to sit in the cold hard chair facing the wall. Anne sat down, the metal cooling the hot blood in her hands. She felt peace now. She took a deep, deep breath, and the fear faded from her eyes.
The gunman now stood in front of her, and he raised the gun so she was looking straight down the barrel; into the eye of death. This was the end. 
I think showing has more adjectives, where as telling is more practical. Not very knowledgeable in that area, but that's what it seems to be for me. 

Edit: What Cassandra said! 
