Clarify image w-h mismatch

docs/specs/assets.md

@@ -20,3 +20,13 @@
 Image formats supported vary depending on the player. Some commonly supported formats are JPEG, GIF, PNG and SVG. 
 
 {schema_object:assets/image}
+
+If the dimensions of the image asset does not match the size given by `w` and `h`,
+renderers MUST ensure image layers referencing that asset do not have any visuals
+exceeding the `w`-`h` size. It's RECOMMENDED they scale the image maintaining
+its aspect ratio and they center it within the $(0, 0)$, $(w, h)$ box.
+
+Even if an image asset does not have any intrinsic size, its contents MUST
+still stay within the `w`-`h` bounds when rendered.
+
+Authoring tools SHOULD export files where `w` and `h` match the physical size of the assets.