In your "Calls to action" in event editor, click on the "Insert Image" menu in the visual editor. 

You will be able to either upload the image from your computer OR add image from a url. 

If you use a url, it must be a direct link to that image, e.g. http://yourwebsite.com/file.jpg will work, but http://yourwebsite.com/file will not.

Once you add your image, in "Link Target" you can set the destination for the image, i.e. where people will go once they click on the image. Please Note, once you add the image, you can not change the link. If you want to use a different link, you have to delete the image and add a new one again.

After adding the image, you take your mouse on the image, and set it to align left, center or right or delete it.

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