Is it forbidden to think about a girl and bring her image to mind respectfully, without imagining her in a sexual situation?

A person is not held accountable for mere self-talk unless they act upon it or speak it. However, thinking and imagining are impermissible if they lead to something forbidden. Dwelling on intrusive thoughts opens the door to temptation and corruption, for these thoughts are the seeds of Satan in the heart, which grow to become intentions, resolves, and then actions.

If the girl is suitable for marriage and you are capable of it, then proceed with it, as it is the optimal solution. The Prophet, peace and blessings be upon him, said: "We have not seen for two who love one another anything like marriage." Otherwise, you must turn away from her and occupy yourself with what benefits you in your religion and your worldly life.