Do a check of what your host is serving down to your browser.
-Click on the image and "view image info"
I use box.com as a repository for my photos because I have low numbers of scans.
It is a business host and not a photo host,
but I think the photos are served with a better fidelity than the mainstream photo hosts.
For my medium format film scans I upload tiffs of 16 bit depth and they can be around 22MB.
When I view those in the Firefox, they are served as .png format, and can be a bit slow to load.
https://app.box.com/s/75d6e107gfday8k09t2rk55bqowr0fg1
This one is scalable by clicking a magnifier and can also be made full screen.
I use an Eizo here with nvidia x server, set to 2560 by 1440.
Typically the 6x7 format is served by box.com to fill the height ( but not the width) like this:
2,048px × 1,634px (scaled to 1,335px × 1,065px)
Then upsampled to fit the 1440 height.
png is a single still format with lossless compression, more modern than jpg
Portable Network Graphics - Wikipedia
The color fidelity of the pngs in the Firefox is usually much better than jpgs ,
but not as good as when viewing the original tiffs on local ssd.
I use gpicview and darktable locally which support Displayport to the Eizo.