To add a clickable URL to any image in your design, click on the Image layer you'd like to add a link to, then click the Settings button in the plugin header, and paste your URL into the Clickable URL Link input field.
This will add a link around your <img> and set the href attribute of your <a> tag (surrounding the image) in the HTML when it's exported from Emailify.
You can apply settings in bulk by opening Settings, then selecting multiple Emailify layers of the same type in Figma. Any setting you change will be applied to all selected layers.