I've never thought about it before. To be honest, I don't think of my thoughts as images or words. They're "just there". It's probably a linked list gone horribly wrong.

I do know that when I'm sitting idle, I can jump through a different topic of thought every second or two. For example, I could be watching television when a Coca-Cola commercial comes on. It might cause me to think about blood sugar, which leads me to running/jogging, which leads me to think about some of the older homes I pass by on my normal running route, which makes me think of the 1940s, which brings me to World War II, now onto the Manhattan Project and the bombing of Hiroshima and Nagasaki.

The commercial hasn't finished yet as I look over to my wife and say "Boxcar".

"What?"
"Boxcar. The name of the plane that dropped the second atomic bomb."
"Okay."
"I think it flew in the first mission with the Enola Gay as the weather plane or something."
"Whatever."

As far as I know, I've always had this thought process of skipping around. It doesn't prevent me from focusing on things, but when I'm not paying attention to any particular topic, I really hop around.