Upload & Sell: Off
i think responsive design will resize it to display correctly to the width of your screen. I viewed my photo website (photo.josephabad.com) on my iphone and displays it to fit to the screen (images are originally around 800px when viewed on a desktop). I do not see any loss of IQ. You do not have to worry about resizing images to accommodate multiple screen sizes.
Also, I hope your images for the web are 72dpi. They should upload no problem on a mobile connection, unless your connection is really bad.
wrong and wrong
you do need to worry about resized images specifically for high density displays or you WILL lose either:
A. image quality
B. screen real estate
and dpi has nothing to do with web images. it's 2013 and people still don't quite understand the difference between dpi, ppi, and when they matter and don't matter