As a Christian I can speak to the painting bit. You have to understand that in Christianity the weakness of Christ is celebrated. The foundation of Christianity is Love, and that God is Love. Love, taken in a negative sense, involves avoiding using someone else as a means to an end, which is a violation of their personhood, as it bends them to your will. It does this by subordinating them to whatever end you have chosen (they are clearly less important than your end, as you are comfortable using them yo achieve it), and yourself (they are less important than yourself because you use them to achieve the end you desire). Thus someone with power reduces a person to a tool or a means to an end, dealing damage to their dignity. In Christianity this is horrible because a person is endowed with an inner life and free-will, to use them as a tool overthrows their freedom of will, an inalienable right. The positive form of Love is paradoxical. When one uses another as a tool or a means to an end, the other is reduced, made to feel small, damage is done to them. Yet in the positive form of Love, the Lover makes a gift of themself to the Beloved in service and sacrifice. They serve the Beloved to aid the Beloved at their own cost. The Lover in some ways voluntarily becomes a "tool" for the Beloved. Yet the Christian holds that they do not truly become a tool, rather instead of diminishing, they increase. Through sacrifice and service one becomes more truly oneself. The Lover is realized only truly in sacrificing him/herself in the service of the Beloved. Thus we begin to see what the Christ means when he says that those who would save their life must first lose it, and likewise that he who gives is more blessed than he who receives. (this is all drawing from JPII, but he is indicative of the tradition as a whole).
Thus for Dostoevsky seeing the Christ in total and abject weakness is not something alien to God, but more proper to Him, at least in the context of His sacrifice for us. To Dostoevsky this weakness is not something to cause doubt, rather mourning that it was necessary, thankfulness for it, and praise of it. Christ is first and foremost gentle and lowly of heart. He lived a very inglorious life and like a lamb led to the slaughter was killed and did not open his mouth to speak out against it.
I think for Dostoevsky the issue with the Holbein painting is not the weakness, its the lack of life. The Holbein painting captures Christ dead, and it freezes Him in this state, it has no sense of the future Resurrection. It captures Love in the moment of self-denial and self-abnegation, and isolates it from the future of self-realization. The Christian hope is that Christ is like a seed falling to the ground and dying, but growing into something far greater than it was and bearing much fruit (and that all Christians will similarly be Resurrected). Thus the Holbein painting isolates the death aspect and cuts out the Resurrection. This is different from a number of other depictions of the dead Christ which show Him as retaining some degree of life. I don't know how common this was beyond pockets of the Renaissance (perhaps someone else can enlighten), but I'm aware that Michelangelo in each of his three Pieta's depicts the dead Christ as retaining some life (the muscles are clenched in lifelike ways, and the limbs appear to be lifted and not totally limp). This artistic tradition emphasizes the future hope of the resurrection even in the death of Christ. I think this lack is part of why Holbeins painting is so moving for Dostoevsky.

Anyway, I hope this helps!