Document image entity (#1802)

Co-authored-by: Franck Nijhof <git@frenck.dev>

docs/core/entity/image.md

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+---
+title: Image Entity
+sidebar_label: Image
+---
+
+An image entity can display a static image. Derive a platform entity from [`homeassistant.components.image.ImageEntity`](https://github.com/home-assistant/core/blob/dev/homeassistant/components/image/__init__.py).
+
+The image entity is a greatly simplified version of the [`camera`](/docs/core/entity/camera) entity, and only supports serving a static image.
+
+To make frontend refetch the image, bump the `image_last_updated` property.
+## Properties
+
+:::tip
+Properties should always only return information from memory and not do I/O (like network requests). Implement `update()` or `async_update()` to fetch data.
+:::
+
+| Name                     | Type  | Default | Description                                                                                                                        |
+| ------------------------ | ----- | ------- | ---------------------------------------------------------------------------------------------------------------------------------- |
+| image_last_updated             | datetime  | `None`  | Timestamp of when the image was last updated. Used to determine `state`.                                                          |
+
+## Methods
+
+### Image
+
+Return bytes of the image.
+
+```python
+class MyImage(ImageEntity):
+    # Implement one of these methods.
+
+    def image(self) -> bytes | None:
+        """Return bytes of image."""
+
+    async def async_image(self) -> bytes | None:
+        """Return bytes of image."""
+```