Does the prayer become invalid if the worshipper imagines forbidden images in his mind, despite seeking refuge with Allah from Satan?

Your prayer is valid even if obsessive thoughts and imaginations prevail, for khushūʿ (devotional humility) is recommended, not obligatory. It is the presence of the heart and the stillness of the limbs. The ḥadīth has stipulated that waswās (whispers of Satan/obsessive thoughts) does not invalidate the prayer, and that it is overlooked as long as one does not act upon it or speak of it. However, if the imaginations lead to the emission of madhī (pre-seminal fluid), the prayer becomes invalid.