2x images for Retina display

apps/block_scout_web/assets/css/_mixins.scss

@@ -198,4 +198,15 @@
       fill: #fff;
     }
   }
+}
+
+@mixin image-2x($image, $width: 100%, $height: 100%) {
+  @media (min--moz-device-pixel-ratio: 1.3),
+         (-o-min-device-pixel-ratio: 2.6/2),
+         (-webkit-min-device-pixel-ratio: 1.3),
+         (min-device-pixel-ratio: 1.3),
+         (min-resolution: 1.3dppx) {
+    background-image: url($image);
+    background-size: $width $height;
+  }
 }

apps/block_scout_web/assets/css/components/_verify_other_explorers.scss

@@ -18,12 +18,14 @@
 	background-size: 12px 12px;
 }
 .blockchair-logo {
+	@include image-2x('/images/icons/blockchair@2x.png', 10px, 16px);
 	background-image: url("/images/icons/blockchair.png");
 	background-repeat: no-repeat;
 	padding-left: 25px;
 	background-size: 10px 16px;
 }
 .etherchain-logo {
+	@include image-2x('/images/icons/etherchain@2x.png', 16px, 16px);
 	background-image: url("/images/icons/etherchain.png");
 	background-repeat: no-repeat;
 	padding-left: 25px;