You can replace all the image from
img folder of template with same name and extension.
1. Change image from html
You can link your own image path to change any image in our template from
src attribute of
<img src="your-img-path" alt="image-alt-text">
W3School can help you to find more in this page.
2. Change image from css
And for backgrounds, change
backgound : url from your stylesheet.
Or just simply check this tutorial to get more info about changing images with css page.