This is documented in Define Questions Page.

The main thing to watch though - thumbnails are multi line height. When you set them up in the Define Questions Page you must provide fillers so that the space on the lines below is occupied, or the controls will clobber each other.

The easiest way to set them up is using the Friendly Page Editor. This is a Drag n Drop environment which lets you move controls - including thumbnails - around, and drag edges to resize them - very quick and easy to use.