RAW formats can/usually have a thumbnail/preview embedded. I convert all of my Nikon RAW files to Adobe DNG (Adobe's RAW format) and I have a Leica camera that

RAW formats can/usually have a thumbnail/preview embedded. I convert all of my Nikon RAW files to Adobe DNG (Adobe's RAW format) and I have a Leica camera that uses DNG natively. When I convert, I can chose to have a full-sized preview embeded. This makes certain software give a faster preview.