Images are limited, in a browser, or in an email application, to 72dpi. That is the maximum resolution for a computer monitor.
However, rather than allowing CC to resize my photos. I always convert them locally to a high resolution format, like Photoshop or TIFF, then I resize them to a max of 125px wide. Those are the images I upload. Then when I link to them, no resizing is necessary, so they look clearer.
Any photo editing application can be used. GIMP (which is free), Photoshop, Photoshop Elements, Corel Draw, etc.