How can I add an object to a scene?

To add an object to a scene, select the menu item Edit -> Add Object or press the Add Object graphic toolbar button in the Object Window.

How can I change an object image?

In the Preview Window toolbar, press the button labeled Image Picker graphic. Click using the left mouse button on the image in the scene that you want to replace with another. The Open File Dialog will open. Select the new image in the Open File Dialog and click OK.