Images are resized not by the theme, but by core of WordPress and your server.
In our demo we are using different image sizes, they are not specially resized before the upload.
We must admit that this kind of problem will occur no matter what theme you are using.
In current layout the image size are cropped to 640x300px, probably you can achieve better results if you upload exactly 2 times bigger image, in this case 1280x600px. If you will change the layout, for example blog post without sidebar, image dimensions will be changed as well, and probably image quality could change either.
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