King Penguins look and behave like Emperor Penguins. But they are smaller and do not weigh as much as emperor penguins do, and they are more brightly coloured. However, according to research that I did on the 24th January 2018, there are several other differences between the two types of penguins, including one difference that I saw but never acknowledged as a key difference.
I remember seeing a picture of a king penguin chick being fed by an adult king penguin more than a decade ago. The king penguin chick had brown fluffy fur but somehow I didn't acknowledge that as a key difference from emperor penguins. I guess because it was only one picture, and had no idea that all king penguin chicks looked like that. Unlike emperor penguin chicks which have grey or ashy white fluffy fur, white fur on their faces, and black fur around their noses and on the other parts of their heads.
King Penguins have longer and less curved beaks, while emperor penguins have shorter and more curved beaks. Juvenile king penguins aren't as colourful as the adult king penguins, though they look very different from the king penguin chicks. King Penguins also have dark yellow or orange colour patches around the throat, but those patches are paler on the emperor penguins, comparatively.
Another significant difference that I was not aware of was that emperor penguins only lived on the Antarctic mainland, while the king penguins lived on the sub-Antarctic islands. So if there's a picture of a penguin on land that doesn't have snow, or indeed any rocks etc. with no snow on them, then they are pictures of king penguins. Based on some videos that I've been watching, king penguins and emperor penguins make similar noises, but emperor penguins tend to make deeper noises than king penguins do.