Trouble sizing featured image

I did use the link to search for former threads, so if I missed this question, I apologize.

I just downloaded this theme and really like it. However, I am pulling my hair out with the featured image feature. I have imported several logos which were sized correctly outside of WP, but one in particular will not shrink to fit in the box on the rolling post page.

I then tried to edit the image within WP, with no apparent result.

I then tried to edit the element code, which neither changed the image nor saved.

All I want is to be able to scale the picture so that it fits fully in the box of the template. I have never had this trouble before, as typically I just resize the picture in another program and upload. I’m confounded.