fix image.html shortcode to use image.html partial

layouts/shortcodes/image.html

@@ -1 +1,7 @@
-<h3>example image</h3>
+{{ $image := .Page.Resources.GetMatch (.Get "src") }}
+{{ partial "image.html"
+   (dict
+   "image" $image
+   "alt" (.Get "alt")
+   "sizes" (slice 320 480 600)
+   "classes" (slice "fill-wh")) }}