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.