Question: Which Form During Japa?
Kindly enlighten me on which form of Lord Sri Krishna I should picture in my mind while chanting Hare Krishna so that I can concentrate on my chanting.
Answer: Krishna's Nama Form
You should focus your mind on Krishna's nama form, i.e. the sound of His name. Every one of His unlimited millions and billions of forms is contained within the sound of His name. So, there is no need to divert your mind from the sound of Krishna's name to meditate on a particular form. The sound vibration of Krishna's name is more powerful for delivering you from material existence than meditating on a mental image of a form of Krishna.
If you are not able to concentrate on the Hare Krishna mantra, beg Krishna to help you. He will be glad to help you fix your mind on the sound of His name.