If you use Magento 2 Blog extension with attached featured images to a blog post but these images are not displayed on the storefront, then most likely the issue is in your theme. A lot of themes provide custom blog layouts and template files that override the original view and can miss some blog functionality like featured images. To check and fix this, please follow steps below:

1. Open this file if exists (if does not exists skip steps 2-3):

/app/design/frontend/[ThemeVendor]/[themename]/Magefan_Blog/templates/post/list/item.phtml

2. Check if the file has the code like "getFeaturedImage". You can find the original code here 

https://github.com/magefan/module-blog/blob/master/view/frontend/templates/post/list/item.phtml
https://prnt.sc/tcl3sj

3. If this code is missing, add it to your custom theme file.

4. Open this file if exists (if does not exists skip steps 5-6):

/app/design/frontend/[ThemeVendor]/[themename]/Magefan_Blog/templates/post/view.phtml

5. Check if it has  the code like "getFeaturedImage", you can find the original code here 

https://github.com/magefan/module-blog/blob/master/view/frontend/templates/post/list/item.phtml
https://prnt.sc/tcl3sj

6. If this code is missing, add it to your custom theme file.

Note: once you've made the code changes don't forget to redeploy your static content and flush Magento caches.