You have mistaken CSS here
background-image: url(background-image:url(/images/topmain.jpg) no-repeat;
It should be
background-image: url('images/topmain.jpg');
background-repeat: no-repeat;
OR
background: url(/images/topmain.jpg) no-repeat;
Update in your snippet
.fullbox {
border: 1px solid orange;
background-image: url('images/topmain.jpg');
background-repeat: no-repeat;
}
<div class="row">
<div class="col-12">
<div class="fullbox">
<h2>plumbing</h2>
</div>
</div>
</div>
Another live example
.fullbox {
border: 1px solid orange;
background-image: url('https://cdn.dribbble.com/users/1928643/avatars/small/b3f1a5ea4d68ff539b8f808ff3c4b8a5.jpg');
background-repeat: no-repeat;
}
<div class="row">
<div class="col-12">
<div class="fullbox">
<h2>plumbing</h2>
</div>
</div>
</div>
To learn more about background-image property and path