No, you really can't tell which "side of the boat" the image is coming from. The best you can do is calculate how far out to each side your sonar ping reaches, then figure it must be somewhere within the fan shape of that sonar ping. Depending on which setting (455kHz or 800kHz) you have dialed up, your fan shaped sonar ping would be either ~45deg or ~75deg wide to either side of dead center (directly below the transducer).
And since your fan shaped sonar ping is only ~6deg thick, your transducer is practically right over top of whatever your screen is showing AS IT COMES ON THE SCREEN. Anything showing on the screen, to the left of the right hand edge of the screen, is "history" ... as in it's already been passed over by your sonar ping. So, depending on your speed, by the time the image registers on the screen & you can identify it .... it may already be behind you, either under the boat or behind the boat.
And since the sonar ping is coming from a single point, and fanning out from there, and the sonar ping can only tell you the distance from that single point to the closest object that reflects the ping back .... the top of the cover is that distance from your transducer, whether it's directly below you OR out to either side. In other words, it may or may not be that far under the water. It could just be that far away from the side of the transducer.
That's one reason I always use the 800kHz setting ... so that I know that if I'm in 10ft of water, my sonar ping spread is about 10ft wide.
( I have the Lowrance Elite 5DSI unit ... bow mounted ... and this 87pg discussion is where I learned about this Down Image stuff : http://www.crappie.com/crappie/fishi...lotter-87.html )