how can we choose most important DCT coeff between all DCT coefficients of an image?

I am working on feature extraction for signature verification,I have splitted my image into 64 segments by vertical and horizontal splitting based on the gravity center point,Now i want to use just x DCT coefficients of each segment How can i choose x most important DCT coefficients between all DCT coeff of a segment? what are the optimal number of extracted coeff which can reconstruct image with acceptable error?

My english is poor,sorry for probable grammatical mistakes