Use this code to insert a link to trigger your pop-up:
<p><a data-target="#UniqueNameForThisPopUp (no spaces or punctuation!)" data-toggle="modal" style="cursor:pointer;color:#337ab7;font-weight:700;">Link Title</a></p>
Place this code at the end of your box's HTML:
<div class="modal fade" id="UniqueNameForThisPopUp" style="display:none;" tabindex="-25">
<div class="modal-dialog modal-lg">
<div class="modal-content">
<div class="modal-header">
<button class="close" data-dismiss="modal"
type="button"><span>×</span></button>
<h4 class="modal-title">Title for your Pop-up Window</h4>
</div>
<div class="modal-body">
<p>Content for your Pop-Up</p>
OR
<p><img style="border:solid 1px #c9c9c9;display:block;margin-left:auto;margin-right:auto;" src="https://imageurl.com"/></p>
</div>
<div class="modal-footer">
<button class="btn btn-default" data-dismiss="modal" type="button">Close</button>
</div>
</div>
</div>
</div>