We recommend that you upload all Speakers' images as the same layout (ie: portrait or landscape) and the same width. If you want your images to be a perfect circle, we recommend uploading square images.
You can edit the images on your website by going to your Event Dashboard > Setup > Website > Edit Website. Then, hover over the image you want to replace and choose Edit Section Image & Spacing from the pop-up menu on the right.
Here is a quick demo on how to edit the images: https://www.loom.com/share/19a05b5ee2a04546b695ef1a9376a01a