Featured Image Resizing (Oxygen Theme)

Hi,

I am using the Oxygen Theme and I am trying to change the size of my featured images (the thumbnails for the posts). They are showing up as rectangles but I want them to be the actual size. I have tried messing with this in the CSS which stretches the images. I have tried changing it in the php which doesnt do anything at all as well.

I was told to go through the FTP so I created an ftp through hostgator but I’m not ure what I should even do with that to affect the images.

Any ideas?

Thanks!