Help Replacing the Banner in a Theme with a Widget

Website: https://www.ekepromo.com
I would like to change/customize one aspect in a wordpress theme (which cannot be change in the themes options) using the style.css or any other file in the editor. I would like to replace the 950px X 300px banner image (the image with various mobile devices and computer monitor) with a widget (WOWSlider) and adjust the size of that widget to 950px X 300px.
Extra important info:
(I do not have html/php experience) I have already successfully changed most of the colors on the theme manually through the style.css file to match my logo. I search the style.css file for “950px” and I found the region of interest( I believe):

#heading {
font-size: 35px;
margin: 20px 0;
text-align: center;
text-transform: uppercase;
}

#slide-container {
min-width: 950px;
height: 320px;
background: url(“images/sback.png”) repeat scroll 0 0 #2E3036;
padding: 20px 0 0;
}

#slide{
margin: 0 auto;
width: 950px;
height: 300px;
}