feat(shortcode): refactor and improve image shortcode (#187)

assets/svg/loading.small.svg

@@ -2,9 +2,9 @@
 <svg width="69" height="69" viewBox="-31 -31 100 100" xmlns="http://www.w3.org/2000/svg">
     <defs>
         <linearGradient x1="8.042%" y1="0%" x2="65.682%" y2="23.865%" id="a">
-            <stop stop-color="#fff" stop-opacity="0" offset="0%"/>
-            <stop stop-color="#fff" stop-opacity=".631" offset="63.146%"/>
-            <stop stop-color="#fff" offset="100%"/>
+            <stop stop-color="#a5a5a5" stop-opacity="0" offset="0%"/>
+            <stop stop-color="#a5a5a5" stop-opacity=".631" offset="63.146%"/>
+            <stop stop-color="#a5a5a5" offset="100%"/>
         </linearGradient>
     </defs>
     <g fill="none" fill-rule="evenodd">
@@ -18,7 +18,7 @@
                     dur="0.9s"
                     repeatCount="indefinite" />
             </path>
-            <circle fill="#fff" cx="36" cy="18" r="1">
+            <circle fill="#a5a5a5" cx="36" cy="18" r="1">
                 <animateTransform
                     attributeName="transform"
                     type="rotate"