Hi everyone, does anyone have a proven method for getting images to show at high resolution?

I normally make my images twice the size that they'll be displayed at to account for different device rendering, especially the iPhone 6 Plus. I size them down in the HTML.

Typically, I save all my images in 72ppi. However, I notice that the image quality has not been consistent in my Litmus rendering tests, especially with smaller images such as logos. Might this be due to the speed with which the device tests are rendered in Litmus?