Personally, I wouldn't use 100% for image width settings, because this encourages end users to not properly scale their images to 1x/2x, thus causing blurriness on the rendering end. It's much more straightforward to use CSS (so you can responsively scale for both older clients like Outlook and support high-DPI devices) and placeholder images that clearly denote the image size in pixels. For example, if 100% of your email's width translates to 500px, I'll put a 1000px image in and set it to 500px for screens that are 500px and above, but 100% for those 499px and below to allow for correct scaling. You get your high DPI for devices that support it, a pixel lock for Outlook, and scaling for smaller screens.