The following are taken from the two sources, 1) from this online page and the book An Introduction to Module Theory by: Ibrahim Assem, Flavio U. Coelho.
In the Abelian Categories chapter in the module theory text on page 157, right after presenting IV.2.21 Definition, the authors states
"Image and coimage may or may not exist, but if they do, then they are unique up to isomorphism (because so are kernels and cokernels).
Also in the reference url page above, the authors present two...