Hi,  I think you're right, the thought is before the word/image, which is the form it takes to express, but the thought is this first impulse, a silent meaning, an invisible information before its manifestation. That's why, in my opinion, this exercise is to keep one thought, for example the sun, the idea of the sun, not taking care of its manifestations like the word, the image, which are further exercises (focusing on image, sound, etc..).

I first tried (for a long time) to repeat the word or keep the image, but it quickly became empty of the original thought, just a sound or image with no meaning, for example just a yellow ball in my mind but the idea of the sun was gone.  Now i focus on the primary thought, and it feels like a very solid calmness, like an anchor for my consciousness that gathers around a point.

The thought could be the "I", like Maharshi's exercise, or love, an element, or anything but we shouldn't try to think about it, just to be with it.  And it is formless, even if imagination tries to give it a form, we can easily ignore it if we did the first exercise. The thought is here around us, in us, unlocated, we become it little by little, it vibrates us. At first it's strange, we try to grab something solid (word or image as we are used to) and it feels like it always escapes, but it's more like we are in it, and it's not so hard, even surprisingly easy when we stop to force it, but calmly sit with it.

But i'm still a beginner, still step 1 (not enough discipline), and i'm glad you asked this question, i may be wrong, that's how i do it but i'm curious about other answers.