The most dramatic moment of truth in Mockingjay occurs at the very end of the novel when Katniss is given the choice to execute President Snow before masses of people gathered outside the square. Katniss wavers in her determination, for she has just learned that President Coin intends to let the Hunger Games continue, using the Capitol children as punishment. At the last moment, she turns and shoots Coin from the balcony instead.
To portray this moment by picture, you could create a symbolic collage of a snowflake, an arrow, and a coin. Rose imagery is often associated with President Snow as well, so it would be interesting to include white or red stained roses as well.