Please change all images to the following format:

<center><figure class="figure"><a class="lightbox" href="/onlinecourse/images/" data-plugin-options="{'type':'image'}"> 
<img class="figure-img img-fluid" src="/onlinecourse/images/" /></a><figcaption class="figure-caption"><strong>A caption...</strong></figcaption></figure></center>
                        

(Images in current articles are already changed)