Document custom image service props

src/content/docs/en/reference/image-service-reference.mdx

@@ -284,6 +284,33 @@ export default defineConfig({
 });
 ```
 
+## Typing custom image service props
+
+<p><Since v="5.16.6" /></p>
+
+If your image service supports additional props in Astro’s `<Image>` component, `<Picture>` component, or the `getImage()` function, you can add types for these by extending the `Astro.CustomImageProps` interface.
+
+For example, to add a custom `blur` prop that your image service supports:
+
+```ts
+declare namespace Astro {
+  interface CustomImageProps {
+    /** Apply a Gaussian blur with this radius to the image. */
+    blur?: number;
+  }
+}
+```
+
+You can expose these types to users by making your image service an [Astro integration](/en/reference/integrations-reference/) and using the [`injectTypes()`](/en/reference/integrations-reference/#injecttypes-option) helper.
+
+Then, users will be able to get autocomplete and type safety for your custom props:
+
+```astro
+<Image blur="yes" src={myPhoto}  />
+//     ^^^^^^^^^^
+//     Type 'string' is not assignable to type 'number | undefined'.
+```
+
 ## Utilities
 
 Astro exposes a number of helper functions that can be used to develop a custom image service. These utilities can be imported from `astro/assets/utils`: