I'm unable to detect walls
(AKA: I never see the laser grid)
If you point straight to a white wall, the camera isn't able to detect it. You need something that is easy to recognise, such as a door or a painting. Just point the camera, move slowly around, and wait until the red grid appears.
How to edit a painting in AR?
First of all you need to detect the wall and place a picture. After that you can turn on the edit mode by tapping the "gear" button on the bottom right side of the screen. Then touch and drag the picture to move around, or pinch to scale. If you want to delete a picture, tap the "delete mode" button, then tap a picture you want to remove.
How to add a picture to the collection?
If you see a picture that you like, and you want to know how it looks in your house, fire up Gallary and tap the plus button. Select the take button to enter the camera mode. Point a picture then tap the shoot button. The editor comes up. You can refine the detected rectangle by dragging the corners, and tap the cut button when you are done. Now you see how the picture looks like in Gallary, and you can add it to your collection. When you go back to your house or office you can place the new picture on a wall to discover if it's perfect for you room.