A simple way to insert objects into an image or movie is using alpha masks. In this example for a counter we used a black box to hide the rest o the number wheel without Dynamic Paint.

To use the compositor, go to “Node Editor”, select “Use Nodes” and “Backdrop”. Copy the node setup, the upper “Render Layer”-input is used to create an Alpha-mask for the merging of rendering and image (you can see the mask in the upper Viewer-window).

Note that we use an “ColorRamp”-node to strengthen the contrast of the mask and an “Invert”-node and a scene with black background and black objects to prevent an unwanted white glow around the inserted objects in the finished rendering.