EasyIndex can create a thumbnail from any post, as long as it can find a suitable image in the post.
To do this, EasyIndex reads the post content and searches for images in the following order:
An explicitly set "Index image". EasyIndex adds an image selection box to the post edit page and you can select an image to use for indexes in much the same way you would select a Featured image.
If no "Index image" has been set, then EasyIndex checks for a Featured image .
If no Featured image is set, then EasyIndex checks for an EasyRecipe recipe and uses the recipe image, if set.
If the post doesn't contain an EasyRecipe with a recipe image, then it checks for an image marked up for rich snippets (i.e. marked up with itemprop="image" - all major recipe and schema.org plugins will specify this)
If there is no image marked up for rich snippets, check for a "post image" that old Thesis themes use
If no image was found above, use the first image that is at least 150x150 pixels.
EasyIndex will search for images and create thumbnails on the fly if necessary. That is, if a post that is to be indexed hasn't yet been searched and a thumbnail created at the time an index is displayed, EasyIndex will search for the image and create the thumbnail when the index is displayed. Once an image is found, it's location (and the thumbnail) is stored, so the process only needs to be done once. When you create a new post, the index thumbnail will be created automatically if and when it's needed.
This means that the very first time you display an index, it could take quite a long time to show the index page since EasyIndex has to find and create the thumbs for every post in the index. To prevent this, EasyIndex allows you to create all the thumbnails in a batch (see Thumbnail Generation) after you've set up (but before you publish) an index.
The above applies to index styles that use sampled posts for the thumbnails. For index styles that have fixed images (the "Pre-set image" styles), you explicitly select which images to use on the Primary Thumbnails tab of the EasyIndex settings page.