I am having a problem understanding what i need to do to fix a Google PageSpeed Insights issue.

The report shows:- 

"Serves images with low resolution Image natural dimensions should be proportional to the display size and the pixel ratio to maximize image clarity. Learn how to provide responsive images."

The image is a Logo in the template header - it is 215 x 49. The mobile test says:-

Displayed size = 215 x 49

Actual size = 215 x 49

Expected size = 323 x 74

I created a seperate image and used that in the mobile template, hiding the main logo for that level.

I have tried changing almost everything, the image to 323 x 74, the size in the image, i have made it bigger, smaller.....nothing has worked. If i do increase the resultion to 323 x 74, it then reports it wants bigger.

How should i fix this? It seems to want a different resolution for the mobile display, i just can't understand what!

Thanks.