You can't. The site applies it's own compression upon uploading. I believe the original upload is used for screening but even if it isn't, it's a function of the upload process that you have no control over. All you can do is upload the best quality you can.
Equipment: 2x Canon EOS 50D; Sigma 10-20 EX DC HSM, 50-500 EX APO DG, Canon 24-105 f/4 L, Speedlite 430EX