When I first joined 123rf (summer of 2005) I was concerned about how much smaller the images were and e-mail them about it. They said that they strip everything (EXIF, profiles, IPTC) out and compress very slightly more to get the small size.
I asked for, and they e-mailed me, 3 of my own images that had been through their image prep process so I could compare them. Perhaps I missed something, but looking at my 20D images at full size I couldn't spot any noticeable artifacts introduced by what they do.
If the "ultra high" is upsampled, then that would be different - I was just looking at the full size versions.