Resizing jpg in header - pic too big!

I need your help, please. I've added a jpg image to my header but it's way too big. It won't scale my beautiful image.

I've walked myself through the CSS tutorials and coding for the header and just can't figure out how to shrink the image to make it fit perfectly in the header. Here's my code:

<=== HTML version ===>

<header class="mountain"></header>

<=== CSS version ===>

.mountain { background:url(header_pict.png); background-position:center; background-repeat:no-repeat; height:260px; background-size: cover; width:100%; border-top: 5px solid white; border-bottom: 1px solid black; }

Mysteriously, it screws up when I use the internal html coding system - like this for example:
<img src="header_pict.png" height="250" width="100%"></header>.

Your help is greatly appreciated.

Laura