To add a basic shape (ellipse, rectangle, triangle), click the Add basic shape icon, place the element holding down the left mouse. Customize the
appearance in the Item Properties tab.
When you also hold down the Shift key while placing the basic shape you can create a perfect square, circle or triangle.
Figure Composer Basic Shape:
The Shape item properties tab allows you to select if you want to draw an ellipse, rectangle or triangle inside the given frame.
You can set the style of the shape using the advanced symbol style dialog with which you can define its outline and fill color, fill pattern, use markers etcetera.
For the rectangle shape, you can set the value of the corner radius to round of the corners.
Note
Unlike other items, you can not style the frame or the background color of the frame.