Not displaying the featured image in a post

Hi,

I’m using the Oxygen theme which I love so much. My issue is that I sometimes have no featured image to add to a post (for example, for some posts about math – http://www.yaronhadad.com/?cat=5) and it therefore displays a grey rectangle instead.

Is there a way to choose a default image for all posts from a certain category? Or instead just to remove the gray rectangles whenever no featured image is selected.

Thanks!