Hi @M72561,
Thanks for reaching out.
One of the features of the responsive design is scaling images, and your site isn’t slow. It’s just according to that tools and it wishes you to follow static sizes (something that never responds).
An example is, because it’s displayed 160x128 on their view (the tool), it wanted you to exactly resize your image to 160x128. But what if it’s viewed on the different device where it scaled up to 300x240? Then your 160x128 image will be blurry since it’ forced to 300x240. Now imagine that on retina devices from Apple, it will be more blurry.
That kind of optimization is only good for non-scaling website, like a dedicated mobile website separate from a desktop website. An example is Facebook mobile site where images are served smaller.
Though, if you insist, you can resize your image to 160x128 using Photo editor like photoshop and upload it as your logo.
Thanks!