If you already had images in your Media Library when you installed Once, you will have to regenerate your image thumbnails.
Follow these steps:
- Go to Plugins → Install Plugins and install Regenerate Thumbnails plugin.
- Then go to Tools → Regenerate Thumbnails.
- Click the Regenerate All Thumbnails button.
It will take some time depending on the number of images in your Media Library. Please do not close your browser, as it will stop the process. You may continue configuring your website in a new browser tab.