Blog template seemingly pulling small image for Featured Image


I am working with Jupiter X and I have chosen the single Blog Post template that uses the full screen featured image at the top. M only issue is that the theme seems to be pulling an 800px wide image and using it to display full screen which causes the image to be pixelated even on mobile.

I want to know if there is a way to have the theme call a larger or the original image to use for the featured image so that it displays higher quality.

You can see an example of what I am referencing here