Add {background-repeat:no-repeat} to each image.
<div data-stellar-background-ratio="0.5" class="image" id="nav1" style="background-position: 0px 0px; background-repeat: no-repeat;"></div>
Curious: Why not assign these styles via a class?
Question: How does one know what values can be used to style a background image?
Answer: This is one of many resources… You should have resources bookmarked for quick easy access.
http://www.w3schools.com/cssref/css3_pr_background.asp