Is it considered polytheism (shirk) or disbelief (kufr) to imagine that Allah is something tangible (like a drawing or an inanimate object) when remembering Allah, and what is the solution for that?

There is no blame upon you for these fantasies as long as you repel them. This is a type of waswasah (whispering of Satan/obsessive thoughts), and we ask Allah to cure you of it. We advise you to constantly supplicate and humble yourself, and to distract yourself from these waswasah. We also recommend that you consult a trustworthy psychiatrist.