Right click on the node or select menu Insert>External object...
   
You will be asked to open a file on your computer (with preview).
Since the object is inserted, you can:
- 
resize it with the handle at the bottom right edge of the object. The proportions will be constrained.
 - 
double click to recover the native size
 - 
double click anywhere on the surface of the object to open it in your favorite editor.