It's like this: every file is encoded and any quality reduction is locked in. We encode many different formats for our various digital partners. If you upload MP3 files (which are not full quality) the files we send to our partners will not only have the inherent quality loss of the format we encode to, but will also have the quality loss of the format you sent us.
So uploading MP3s is kind of like taking a picture of a picture with a digital camera and emailing it to us. Then when we encode it to send it to partners, it's like we print out the picture of a picture, take a picture of it with a digital camera, and email it to them. It makes much more sense if you send us the original picture, i.e. FLAC or WAV files.