It was prophesied that Krishna will take birth from Devaki and kill Kansa. This picture is about Kamsa trying to Kill Devaki so that Krishna is not born.
Below is the complete story:

The Advent of Lord Krishna

Krishna was born in a tense historical period preceeding a devastating war. The warring factions built up so many weapons that the burden on the earth became unbearable. Finally the goddess of Earth took the form of a cow and prayed to Lord Brahma for relief. Lord Brahma called all the demigods to the shore of the Milk Ocean to hear Mother Earth and to worship the Supreme Personality of Godhead, Lord Vishnu. Lord Brahma fell into trance reciting the Vedic hymns known as the Purusa-sukta and heard the voice of Lord Vishnu. Then he announced, "O demigods, hear from me the words of God. He is aware of the distress on Earth and wants you demigods to incarnate as sons and daughters in the Yadu dynasty. The Supreme Personality of Godhead, Lord Krishna, will personally appear as the son of Vasudeva. Therefore you will all have the benediction of joining the eternal pastimes of Lord Krishna."

Lord Brahma consoled the cow and sent her home, then returned to his planet, Brahmaloka. The demigods then began to take birth in the Yadu dynasty, awaiting the appearance of Lord Krishna. The members of the Yadu dynasty, headed by Vasudeva and Devaki, along with their friends, relatives and well- wishers were all demigods. The residents of Vrindavana, headed by King Nanda, Queen Yasoda and Queen Rohini, were also demigods.

King Kamsa was another relative in the family, however he was not a demigod. He usurped the throne of his father, Ugrasena, and put him in prison. When Devaki, a member of Ugrasena's family, married Vasudeva, she received a large dowry of elephants, horses, chariots and servants. After the wedding, Kamsa took the reins of the wedding chariot and started to escort the couple home. Along the way, a voice from the sky addressed him: "You foolish king, the eighth son of Devaki will kill you!"

Kamsa pulled Devaki down by her hair, drew his sword and prepared to kill her on the spot, but Vausdeva begged for his bride's life and promised to let him kill the eighth child, so that the oracle would not be fulfilled. Kamsa agreed to spare her life, but locked Vasudeva and Devaki in a stone prison. Thereafter, he mercilessly killed the first six sons of Devaki. Devaki's seventh son miscarried but mystically transferred to the womb of Queen Rohini in Vrindavana. This became Krishna's older brother, Balarama. Soon thereafter, Devaki became pregnant with her eighth child

[Source: http://www.surrealist.org/gurukula/storymatters/krishna.html].