Originally posted by Rondec but I find that uploading them just really seems to soften them compared to the originals
Server image compression (GDLib compression more than likely) - most high volume servers do this, usually in the 8~10% range to save on bandwidth. Most of the time the average person with the average monitor can not tell - obviously you notice it as you have seen the originals...
EDIT: PF compresses when images are uploaded to user galleries:
CREATOR: gd-jpeg v1.0 (using IJG JPEG v62), quality = 87
You will see that when viewing a user gallery photo, it indicates that the image was compressed to 87% from the original, so 13% of the color and graphic structure information was truncated from the file...