The main key is that you can look at any file as a string of bits! And apply Huffman coding at whatever granularity you like. For text files, you're essentially

The main key is that you can look at any file as a string of bits! And apply Huffman coding at whatever granularity you like. For text files, you're essentially applying it at the byte level (since each character is a byte (in ASCII, anyways)). For images, you might have one byte per colour channel, RGB. Then you can apply Huffman coding at the byte level, or even at the 3 byte level to operate on entire colours as opposed to channels.