Cover images (or thumbnails) are made outside of Boom and uploaded. Canva, Powerpoint or any editing software will work. Your image size should be at least 300 x 300. To add a cover image once it is created and saved as .png or .jpg follow these steps:
1. Open the Details box (top menu bar).
2. Click on the image icon in the upper left of the Details box.
3. Select an image or click upload if you need to upload your image first.
4. Now you have your cover thumbnail.
You can't delete a thumbnail once it is uploaded, but you can change it.