bfi_thumb creates new file names that are not picked up by WP Retina 2x. For example, you may have an image file called your_image.jpg and WP Retina 2x may have prepared another called email@example.com to serve on retina devices. But when you bring bfi_thumb to the party as well, it’ll create it’s own file e.g. your_image_resized.jpg. When your page is loading WP Retina 2x sees your_image_resized.jpg, it doesn’t have a retina-equivalent prepared.