How does Behrman's last painting become a masterpiece and save a life in "The Last Leaf" by O. Henry?

Behrman's last painting becomes a true masterpiece because of its realism. It was so true to life that it fooled everyone, but especially Johnsy, for she was the one he had done the painting for. If the painting had not so masterfully depicted the last leaf holding onto life on the withering ivy vine, Johnsy would not have been convinced and, in her despair, she would have given herself over to death.

In addition, the painting is remarkable because it becomes evident later that the frail old man had gone outside to face the most atrocious weather conditions to paint the leaf. The fact that he could even paint so precisely in such a storm makes his rendition so much more phenomenal.

Furthermore, in spite of his frailty and the prevalence of a deadly disease, Behrman was prepared to put himself at risk of infection and exposure by performing a remarkably sacrificial act to show how much he cared for Johnsy and human life. As it is, being out in the freezing cold and rain led to an infection which his fragile body could not fight and he was ravaged by pneumonia and later died. Of all the acts that he performed, this stands out as the greatest and accentuates the fact that his simple work of art is a masterpiece. 

Behrman did not expect any reward or praise for his work. He realized that saving Johnsy's life would be a reward in itself and he more than admirably succeeded in doing so. Johnsy had put all her hope for survival in that one last leaf she saw clinging to the vine. When she saw the vine losing its leaves, she started counting them and stated that her life was ebbing away just as the leaves were dropping.

Johnsy determined that when the last leaf had fallen, she would die. As she told Sue:

Leaves. On the ivy vine. When the last one falls I must go, too.

I want to see the last one fall before it gets dark. Then I'll go, too.

When Sue reported this to Behrman, a downstairs neighbor, he was quite upset and saw Johnsy's remarks as foolish. He then decided to perform his most magnanimous deed. Johnsy's life was saved when she believed that the fragile leaf was clinging to life and, if it could survive against all odds, so could she. The inspiration she got from the leaf gave her hope and she decided to live. 

"I've been a bad girl, Sudie," said Johnsy. "Something has made that last leaf stay there to show me how wicked I was. It is a sin to want to die."