Featured Image crop to low resolution

Hi,

I have used the ‘featured image’ for a team member page but realised that the resolution is low and the image have been resize to 350×350/250×250 instead of the original quality at 800×800. http://bdw.im/newsite/about-us/ and http://bdw.im/newsite/team/kevin-luten/

Is there a way to make the quality of the image better?

Thank you,
Ronald