There are two ways of moving and two ways of resizing your images once they are on a wall, mostly dependent on where you are clicking.
Moving while keeping the current size: Click and drag on an orange edge
Moving while allowing the canvas size to change: Click and drag from within the image. The blue square indicates where the canvas will be dropped, and the size it will revert to.
Resizing while deciding: Click and drag an orange corner if you're still in the design process and haven't decided what size will work best
Resizing to an exact measurement, or create a custom size: Use the pencil icon to enter the size you need. Press "Enter" to apply.
Want to see it in action? Check this out!