I would like to share the following info on 500px.

To taste the water I submitted a few dozen images to 500px.
I had assumed a decent level of security so I took no precautions.
I uploaded 3000 pixels (bigger side) since that was the minimum requirement to sell.
I had not yet realized that you could upload low res and then upload hi res in the market.
The preview image was about 700/1000 pixels (bigger side) when seen on my screen, so it looked ok.

I then stumbled using Google image search on 500px pictures which were MUCH bigger.
Since I doubt that Google will resort to shady behaviour, this looked like something wrong in their site.

If you right click on the image it tells you that the image is copyrighted and do not allow you to download.
But we all now that is a fake protection.

I started looking at the source code of the page displaying my images. The link to the image was in plain sight. No attempt to obfuscate the url with some fancy javascript footwork as many do.
Not a big problem since you assume that the link is to a low res picture.
Alas, this is not so.
In my case the 3000x2000 picture has ,yes, been downsized but to 2048x1370.
This is WAY to big for showcase purpose. Is is bigger than a full HD screen (1920x1080).

I really wonder why the feed a 2.8Mpx asset to the browser.
This rise some questions.
1- are they grossly incompetent ?
2- are they aware but have their reasons (which I would like to know) ?

The only reason I see is that they did it for 4K monitors.
If this is the case they should have some code to feed the browser according to screen resolution and not scatter 2.8Mpx media everywhere.

Hope this helps.