Personally I like the koala and agree you would want to get another of the same size, same series to hang next to it. It is necessary because the crib is long and wide but the print is tall and narrow. Two prints visually combine to be a long and narrow effect, mimicking the crib and making the proportions much better.

With all that said, I wouldn't hang large framed art there at all. I'd consider maybe some decal art or a mural or something. I don't know how old your son is, but by the time most babies are about a year old they will be able to reach anything that big and will be attempting to pull it down. Also I don't know if you have one of those attached changing tables but they speed up the age at which they can get up/out and reach things. My son found my grandson in the middle of the night sleeping on top of the changing table attached to his crib at nine months, even with the mattress on the lowest setting. Terrifying lol.