add note on local image

Author: PatrickFarley

articles/cognitive-services/Computer-vision/how-to/image-retrieval.md

@@ -49,6 +49,8 @@ curl.exe -v -X POST "https://<endpoint>/computervision/retrieval:vectorizeImage?
 }"
 ```
 
+To vectorize a local image, you'd put the binary image data in the HTTP request body.
+
 The API call returns an **vector** JSON object, which defines the image's coordinates in the high-dimensional vector space.
 
 ```json