There are two ways to do this. First, you can upload the graphic as normal, put it in a playlist and set the duration to the maximum 3600 seconds (60 minutes). That way you will only see a very brief (less than 1 second) image transition once per hour when the playlist refreshes. For most people this is the easiest way.
You can also upload your image as an overlay, in which case there will never be any image transitions.