A bit unsure what you are looking for, hopefully this will help:
You can put a photo on hinges, either top or side, and put the keys behind the photo. A canvas would do nicely as it has a built in space.
Hi Luc: Are you looking to hide a house key? If you hide it inside, well, that defeats the purpose:) If you want to hide it outside, that's easier. I had a dog house in the back yard, and when I wanted to hide a key, I would hide it in the dog house. I put a cup hook on the inside, right over the opening, the inside front, and hung the key there. If I was going to be gone when the kids got home, I'd leave a note saying "The key is at Kelly's house" Kelly was the dog:) They knew where to go and get it.