I know what you mean. I wonder whether it's useful to distinguish thinking from that "mind's eye" imaging capability that I believe you're describing. They might be separate things. It's pretty evident that a cat or a dog, or even a fish, has memory. Is memory the ability to conjure things in the mind's eye? If remembering one image reminds us of other images, then language-free thought might be moving through a chain of images. Perhaps humans are good at creating such chains in their heads, while a fly or an ant can't do much more beyond matching a memory to their actual instantaneous sensory input, and reacting accordingly. Substitute "image" with symbolic memories, and that might be the start of abstract reasoning.

In other words, there's probably basic consciousness at one end of a spectrum, and thinking evidenced by language on the other end. Somewhere between those two, one might draw a line between thought and mere conscious awareness.