You may think my statement was extreme, but you should measure the difference before you put me in the loony bin.
Actually, images, if hosted correctly, rarely get downloaded more than once, so their impact on page rendering is not as large as you would imagine. And, obviously, images can add a significantly value to your content.
Rounded corners, implemented CSS techniques, are rendered in the browser. When a page only has a few of those "boxes" with rounded corners, the performance cost can be discounted. When you have many, depending on the technique used, their rendering (which happens every time the page is loaded) can add 20% to 30% to the total rendering time. And, rounded corners don't add a lot of value to your content.
That's all I'll say in my own defense, since you already assumed I am one of those bandwidth/CPU preservation fanatics.