It looks perfectly sharp on my end, both the container and the image sizes are 400×300, please check this: https://i.imgur.com/tdinsgv.png
The images on the screenshot are in native resolution.
I suspect you might be having an issue with the browser? Maybe the magnifying on the page is set above/below 100%, make sure to check that. I also recommend trying without browser extensions, to make sure nothing conflicts there.
