This is a very interesting topic.
I think largely in pictures, but not so much with abstract concepts such as "interesting". With abstract concepts I generally just think of the ways that a given thing can fit into the given abstract category. For example, with my above statement "this is a very interesting topic," I think of all of the different examples of thought processes that I read about in this thread. In other cases, I will imagine myself experiencing the abstract concept. For example, when I read "I do not understand how somebody can think in pictures," I imagined myself with a thought bubble with nothing in it, which I guess is also an image.
With concrete concepts I believe I think pretty much exclusively in images. When I read "think of a cat," I imagined a cartoonish cat licking its paw. When I read "Now picture a cat," I thought the exact same thing. A few minutes ago, my girlfriend asked me if I wanted her to throw some of my clothes in the wash with hers. I imagined the clothing item in the state it was in at the time, and then I imagined what they would look like if they were washed. I also imagined wearing them dirty, leaving them on the floor, and washing them at my house. I replied with "yes," and as i did so, I imagined her throwing them in the washing machine and then coming out clean. I make most of my decisions by visualizing what the situation would look like based on any of the different choices I can make.
I also watch processes happen in my head while I am reading about them, or while they are being explained to me. I can't understand how something works unless I imagine it happening in my head. This is perhaps why I am good at math. I have to imagine to some extent the actual physical process the math problem is explaining in order to solve the problem.