If your father is of sound mind you can simply have him return the painting to you. He has a right to transfer the painting to you while he is alive. From what you state, his new wife has a claim only to furnishings that are in the house at the time of death. A careful review of the prenuptial agreement will be necessary before I can render an opinion on how to proceed.
If you are living in the NYC metro area, please call me to discuss the matter at greater length.
Leonard M. Weiner, Esq./Divorce Solutions