The images are not displaying, what is wrong?
First, make sure the options are correct
On the General Options -> Image Options panel, make sure you have not disabled the images, and the sources are set correctly:
Are the images only in the post/page content?
If you don't use featured images, but the images are in the post content, then they might be a part of a shortcode or gallery or something else, which is not directly stored in the database.
In this case enable the Execute shortcodes when looking for images in content? option on the General Options -> Image Options panel:
Are the permissions correct to store the thumbnails?
The search parses the first/second/third image and the featured image from the post/page content. Check if there is an image in the content.
Make sure, that the following folders exist and are writeable (chmod to 755 or 775):
If you don't know how to change the permissions on your server folders, then this Siteground article will help you out.
Make sure, that the CURL or the “fopen url wrapper” is enabled on your server – remotely hosted images require this. (most of the servers have CURL already installed)
These errors are most likely reported by the error reporting module in the plugin, but in some cases you need to check manually!
Changing the image handler
By default the BFI Thumb image library is used to retrieve images. If that's not working you can change this on the Cache Settings submenu: