I think you could, but indirectly. Try editing the high-res JPEG version of the photo then save it as PNG (Paint can do that). If you can edit the DNG file and save it with high resolution, it'd be a better option.
I took a shot from my phone and cropped a portion of the high-res JPEG image so that the cropped image will have twice the pixel width and length of the usual requirement for cover photos (so that they'll look good).
For something that I haven't used yet, try 1Shot or SoZoom. Both are on WPStore.
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