FLAC is meant for audio only. But you can open a BMP file as raw audio (8-bit unsigned mono) using a sound-editing tool, then save it as a FLAC file. Since FL

FLAC is meant for audio only. But you can open a BMP file as raw audio (8-bit unsigned mono) using a sound-editing tool, then save it as a FLAC file. Since FLAC is lossless, you get your original data back when you decompress it. It just happened to work particularly well on a certain grayscale image. Because only an 8-bit grayscale image could potentially be similar to 8-bit mono unsigned audio, I wouldn't try any other image format.