To use one of the custom templates provided simply add an image block to your site. Once added click the image to show the edit dialogue box and select custom template from the list.

Select the custom template that you want. Save.

Points to note:

Roundy custom template works best with images that have been cropped square. Rectangular images will display as ovals.

Backstretch works best with images that are between 800px and 1000px wide. 

Roundy & Borders styles can be applied to images which have been added through the Content block too. Just click on your image and then on the small image icon in the toolbar. When the modal box appears, click the second tab that says Appearance & select the style that you want by using the Class select box.