Tip: Are you a non-native English speaker? I have just finished creating a Web App for people who enjoy to learn by reading. Make sure to check it out; it's free to sign up.

The equivalent expression in many languages employs a preposition translated as “on” in most other contexts (e.g. “sur” in French). In English, however, the correct preposition is “in”:

The pattern is the same no matter what word we use for the visual media (e.g. image, photo, picture, drawing):

We only use “on” when we mean that something is on top of a physical object; for example “there’s a cup on a photo” means that the cup lies on a photo. However, “on” can also be used when one thing is part of the top layer of another thing. This can be a little confusing for words like “postcard”. You would say:

The reason is that a postcard is the piece of paper itself, not what’s printed on it (unlike the word “picture”, which refers to the actual visual content). What you mean is in fact: “There’s a house (in the picture that is) on the postcard.”

Similarly, if you saw a picture of a man drawn on an envelope, you wouldn’t say that there’s a man in an envelope, would you? The (picture of the) man is on the envelope.

By the way, have you already seen my brand new web app for non-native speakers of English? It's based on reading texts and learning by having all meanings, pronunciations, grammar forms etc. easily accessible. It looks like this: