Made display thumbnail generation use original data if smaller

Thumbnail generation would sometimes create a file larger than the
original, if the original was already well optimized, therefore making
the thumbnail counter-productive. This change compares the sizes of the
original and the generated thumbnail, and uses the smaller of the two if
the thumbnail does not change the aspect ratio of the image.

Fixes #1751

app/Uploads/ImageService.php

@@ -254,7 +254,16 @@ class ImageService extends UploadService
         } else {
             $thumb->fit($width, $height);
         }
-        return (string)$thumb->encode();
+
+        $thumbData = (string)$thumb->encode();
+
+        // Use original image data if we're keeping the ratio
+        // and the resizing does not save any space.
+        if ($keepRatio && strlen($thumbData) > strlen($imageData)) {
+            return $imageData;
+        }
+
+        return $thumbData;
     }
 
     /**