Regenerating Thumbnails

Sometimes, after making a large change to a site such as changing the active theme, you might find that there are some issues with the responsive images; this is due to the new theme's code having different thumbnail requirements, so in this case you will need to regenerate the thumbnails.

This is an automated process accessible by clicking on the Tools & Plugins option in the left-sidebar and then selecting the Regenerate thumbnails section. On the next screen, click on the Regenerate Thumbnails button to begin the process. 

Note that this process may take some time if you have a large number of images on your site, so please be patient.