How does using DCT help in image compression
If you take an image as is (especially a photograph, as opposed to line art, a document scan...), there's very little redundancy in it for Huffman encoding to do its work - because the image contains many small fluctuations and details which are not perceptible to the human eye but nevertheless present in the data. DCT followed by coefficient truncation/quantization eliminates the barely perceptible details and creates redundancy for Huffman compression to work on.