Most of the imagery occurs when he shoots the elephant for the first time, and then as the elephant slowly dies.
"a mysterious, terrible change had come over the elephant. He neither stirred nor fell, but every line of his body had altered. He looked suddenly stricken, shrunken, immensely old, as though the frightful impact of the bullet had paralysed him without knocking him down. At last, after what seemed a long time – it might have been five seconds, I dare say – he sagged flabbily to his knees. His mouth slobbered. An enormous senility seemed to have settled upon him. One could have imagined him thousands of years old."
Then after he fired his last two shots into the elephant, he describes the blood and the agony of the elephant. Orwell does this so the reader can see the pain of both the elephant and the regret of his own as he fell into the "peer pressure" that the Burmese had over him.
"The thick blood welled out of him like red velvet, but still he did not die. His body did not even jerk when the shots hit him, the tortured breathing continued without a pause. He was dying, very slowly and in great agony, but in some world remote from me where not even a bullet could damage him further."