You can customize blog preview image and the blog cover page by clicking "Background" located near the top right of the blog post editor. 
Note:
- The blog post preview and your cover page will always be the same. We don't have a way to make your blog preview images different from your blog cover page as of now.
- If you have faces on your cover page, don't worry! We have a smart way to detect faces so it shows up on your blog cover page.
In case you need assistance, shoot us a line at support@strikingly.com or chat with us.