Improve layer typings

src/ol/layer/BaseImage.js

@@ -37,7 +37,8 @@ import Layer from './Layer.js';
  * property on the layer object; for example, setting `title: 'My Title'` in the
  * options means that `title` is observable, and has get/set accessors.
  *
- * @extends {Layer<import("../source/Image.js").default>}
+ * @template {import("../source/Image.js").default} ImageSourceType
+ * @extends {Layer<ImageSourceType>}
  * @api
  */
 class BaseImageLayer extends Layer {