improve comments in image styles

less/scaffolding.less

@@ -32,10 +32,12 @@ a:hover {
 // Images
 // -------------------------
 
+// Rounded corners
 .img-rounded {
   .border-radius(6px);
 }
 
+// Add polaroid-esque trim
 .img-polaroid {
   padding: 4px;
   background-color: #fff;
@@ -44,6 +46,7 @@ a:hover {
   .box-shadow(0 1px 3px rgba(0,0,0,.1));
 }
 
+// Perfect circle
 .img-circle {
-  .border-radius(500px);
+  .border-radius(500px); // crank the border-radius so it works with most reasonably sized images
 }