Featured Image Blurry as Page Header Images

Hi, I was a pro user of Catch Inspire and am currently experimenting with Euphony. I have spent a couple hours trying to figure out this problem. I have featured images configured to display as page header images. However, it seems my featured image uploads are being auto-resized and/or displayed in different resolutions. I am seeing different image sizes displayed on a desktop computer compared with an iPhone. I have tried resizing my images to many different sizes without success. I would like to have the entire image displayed or at least one that is not so low quality.