I would have though that reducing the file size would only increase the speed with which the photos would open: it should not impact photo quality unless it became pixilated by excessive file size reduction. I can only assume the photo was scanned (by someone) for upload, as that would impact both the quality and explain the distortion.
I don’t know anything about websites: I have always assumed that photos are simply uploaded and what you see depends on the quality of the device you are using to view that website. It is possible that the website uses some form of colour rendering or colour optimisation and that would affect the image as seen by the viewer. I doubt if it is altered in this way, so do write directly to Open Rent for their opinion.