A Logo, or image is the larger picture that appears in the main detailed view of your advertisment.

It should be sized at 300 pixels x 300 pixels.

An Icon or Thumbnail is the small image that is displayed in the business drectory preview pages, like shown below.

It should be sized 80 pixels by 80 pixels

You can choose whether to have the same or a different picture in these two locations, but you still pay for each image individually.