The website needs to understand how to serve images to retina (or the windows equivalent - hiDPI) machines. Many are not. Many websites need some extra code: for example this: Jao's photo blog: A much simpler way to serve retina displays the right images . Image sites like flickr, smugmug, 500px etc, all do this correctly if you upload large enough images but those are exceptions. Also, even if you are using a website that understands how to do this, your browser might not be new enough to ask for the higher res image. Only newer versions of Firefox and Chrome do this. Safari has always done it right since the retina machines came out and on iPads and iPhones.
My guess is that the website you are using is not doing this right. Which is not surprising as these displays are still fairly uncommon. So your website probably resizes the image at the size it is thinking it is displaying on normal screens.
Lastly, high sharpening is usually too much. Also think about the size you need. A 13" retina has a 2560-by-1600 pixel resolution. A 1280 pixel wide image will look good when it takes up about 50% of the width of your display. At that size it will be about 1 pixel in the file per pixel on the screen. If it is displayed larger, it will look unsharp. So think about how wide on the display it will be shown and determine from that your maximum size you need. Preview is not a good gauge as it by default doesn't show 1:1. You need to change the setting in the preview preferences->images to be "1 image pixel equals 1 screen pixel" and then do a command-zero on your exported file to see what it actually looks like. Also, this only works correctly if you set the resolution of your screen in System preferences to "best for display"
That was a great answer. I will see if they can do something about the coding on the website. Now, what would you recommend that I export for the website? All of then images that I have uploaded before I got this new MBP that are sized at 1280px take up the whole screen on the website. To me it seems like either the website "thinks" the display is still 1280px? I just don't quite understand how my images can be 5363px × 3352px and fit within the screen and look great as opposed to exporting at 1280, them still take the whole screen up, even after pressing command-zero, and look out of focus.
>To me it seems like either the website "thinks" the display is still 1280px?
That's actually exactly what happens. Your browser tells the web server that it has a display of 1280x800. If asked it will also say that it's resolution factor is 2.0 or twice as many pixels as the formal size. This seems silly but is done to prevent text from becoming incredibly small.
>as opposed to exporting at 1280, them still take the whole screen up, even after pressing command-zero, and look out of focus.
If you scale 1280 pixels over a 2560 pixel wide display, you will see that it is not quite sharp. The web browser is just doing what its told and you can't make it display 1:1 easily unfortunately.
That makes sense. Thank you for the insightful answers! I guess I'll just have to wait and see what the website is going to do!