Don't try to generate double-density thumbs for small images

models/Image.php

@@ -74,6 +74,10 @@ class Image extends Asset
 		{
 			$thumbnail = new Thumbnail($this);
 			$image_urls[1] = $this->getThumbnailUrl($width, $height, false);
+
+			// Can we afford to generate double-density thumbnails as well?
+			if ((!isset($width) || $this->image_width >= $width * 2) &&
+				(!isset($height) || $this->image_height >= $height * 2))
 				$image_urls[2] = $this->getThumbnailUrl($width * 2, $height * 2, false);
 		}
 		else