It would be great if it were allowed to assign an image to an object. That way maps would contain much more information at a glance than a simple color outline on objects 
Images as objects are called “Tile Objects”. Just create an Object Layer, select a tile, select the “Insert Tile” tool, and click on the map to add such objects. They can be resized and rotated just like other object types.
Ok thanks! Good enough. Is there a way to prevent the image from resizing??
No, you’d just have to avoid hitting the resize handles. If this is hard, the only situation where you can suppress it is if you want to drag objects around, in which case you can hold the Alt modifier key to force the drag operation rather than a resize operation.
A way to reset the image back to its native size would also be helpful, but it doesn’t exist yet.