Hi,
If you want to make the checkbox not checked by default then add this in user.config:
<add key="ResizeEditorUploadedImages" value="false" />
If you are uploading images that are optimized then you should uncheck the resize option.
Our resizing logic doesn't have special "optimization", it just sets the pixel size, but in general it is useful because most users do not have technical background and they upload full size images from their mega mega pixel camera and this does help get the image to something useable in a web page. Before we had the re-sizing I would see web sites with huge multi megabyte images that make the page load slow but the user resized it with html width and height on the img element. So it is a big improvement over raw images the user may upload.
You can also set the default resizing parameters:
<add key="ResizeImageDefaultMaxWidth" value="550" />
<add key="ResizeImageDefaultMaxHeight" value="550" />
But again, anyone who is smart enough to optimize the image before uploading should just uncheck the box to prevent resizing.
Hope that helps,
Joe