Unfortunately I don't know if CSP has an option like this, but Photoshop allows you to choose how to resample the image as it's resized. Different things work for different images.
Otherwise, there isn't much you can do other than try to sharpen it after the fact. Honestly though? I don't think many people will notice or care, especially if they only see the very slightly blurry version. I understand the need for it to look crisp but it isn't something to worry a ton about 
P.S. I don't even bother doing PNG for web posting, the high quality JPGs looks exactly the same these days and are smaller in file size.