improve size classes

static/css/snap-gallery.css

@@ -27,12 +27,16 @@ figure img {
   max-height: 100%;
 }
 
-figure.small img {
-  max-height: 150px;
+figure.sm {
+  max-width: 30%;
 }
 
-figure.medium img {
-  max-height: 225px;
+figure.md {
+  max-width: 50%;
+}
+
+figure.lg {
+  max-width: 70%;
 }
 
 figure.pull-right {