What other properties are in the "entry.image" object?
When you reference a relationship field (ie: Entries field, Assets field, etc), you will automatically get an ElementCriteriaModel to represent the "value" of that field.

Since entry.image is giving you an ElementCriteriaModel, you need to specify exactly what you want that ECM to do. If you try to use the ECM without specifying .first (or .last, or .total), then by default it will assume you meant .find. Which is obviously not what you're trying to do in this case.

I highly recommend reading my answer on this other thread... It's not quite a duplicate of your question, but it covers a lot of the same ground.