You can easily upload an image of a blueprint to use as a background for your diagram. Once you have uploaded your image it appears on the canvas behind any icons you have in your diagram. You can adjust and resize your floor plan image as needed by clicking on the image and dragging one of the corners.
If you want to prevent accidentally moving your floor plan image once you have it sized and positioned where you'd like, just right-click on it a select Lock. That will lock the image in place, allowing you to pan around and easily move icons on top of it. You can always unlock it by right-clicking on the background image and selecting Unlock.