You can start by observing the general mood of the picture. For example, if it looks gloomy, you could start a story about a lost traveler in a desolate place. Just let your first impression guide you.
The colors. If it's mostly blue, it could be a story about the ocean or a cold place. Maybe a sailor lost at sea. If it's red and orange, perhaps it's a story about a fire or a passionate event.
Just look for any prominent elements. Say there's a single flower in the ambiguous picture. I'd start a story like this: There was once a magical flower that grew in a forgotten garden. It was the only one of its kind. People from all around the world heard rumors about it and many adventurers set out to find it. One day, a young girl with a pure heart stumbled upon the garden and the flower.
In the beach picture, there are waves crashing onto the shore. A little boy is building a sandcastle. His parents are sitting under an umbrella, reading books. There are also some shells scattered on the sand. It's a very relaxing scene.
There is a picture on the wall. It depicts a couple kissing in the rain. This picture was taken by a passer - by. The couple had just had a big fight, but then they realized how much they loved each other. In that moment of making up, as the rain poured down, they kissed passionately. And the picture freezes that moment of love forever.
Once upon a time, there was an old, haunted house. A photographer decided to take pictures there at night. As he snapped a photo in the attic, he saw a strange figure in the viewfinder. When he developed the picture, there was a blurry white shape that looked like a woman in a long dress. He felt a chill down his spine. He later learned that a woman had died in that attic many years ago.
There was a picture in an old attic. It showed a dark forest. One night, as I stared at it, the trees in the picture started to sway. I rubbed my eyes, thinking it was a trick of the light. But then, a shadowy figure emerged from behind a tree. It was a faceless man in a tattered cloak. His hands reached out of the picture towards me, and I could feel a coldness in the air. I quickly turned the picture face - down and ran out of the attic, never to look at it again.
Colors are key. For example, warm colors like red and orange can convey passion or energy, which is part of the story. Also, the composition matters. If the main subject is in the center, it might show importance. And facial expressions, if there are people in the picture. A smile can tell a happy story.