How can I experience the pillar of Ihsan (excellence) in acts of worship and rid myself of involuntary images and thoughts that cross my mind during them, especially when repeating "You alone we worship and You alone we ask for help," and do these images constitute Shirk (polytheism)?

What is presented to you is pure waswasa (whispering of Satan/evil suggestion), and its cure is to ignore it and pay no attention to it. It will not harm you nor affect your faith, and your dislike for it is proof of the soundness of your faith.

You must occupy your heart and mind with contemplating the greatness of Allah and His attributes, and being mindful of His awareness of you and His encompassing presence around you, so that this leads you to the station of ihsan (excellence in worship).