Weird siege gun

[Serpent77]

What is that strange thing attached to the siege gun image (see attached picture). On what real life model this generic picture is based on?
I browsed the web but could not find any corresponding artillery piece. That round thing looks a bit unnatural. How about editing the image and deleting it (unfortunately i don't know how to do it myself) or replacing it with a new model based on the famous Austrian 305 mm howitzer "Skoda 30.5 cm Morser" (see attachment) or English 9.2 inch howitzer? Just want the great game to look even better. For instance the generic pic for heavy artillery is perfect to my mind. I really can't enjoy the game if that little details keep catching my eyes constantly.
So, are there any entusiasts familiar with graphic modding?