General Options -> Image Options
On this panel you can turn on/off the images in results, determine the image sources and resizing methods.
General Options -> Image Options -> Primary image source / Secondary image source / ...
The search will parse through the given image sources starting from the primary source. As soon as an image is found it returns that image to the main thread and displays it on the results list.
There are currently 6 different image sources to choose from:
Post content parsing
Post excerpt parsing
Custom field as image source
Page screenshot (WARNING - THIS IS AN EXTERNAL SOURCE, IT CAN BE VERY SLOW)
The default image
General Options -> Image Options -> Custom field containing the image
If you have choosen for any image source a custom field, then you must enter the name of the custom field which holds the image url.