The main media for an Essential Grid item is usually
represented by an image, but can also be other media content
such as video.
Whether you're using a product-based grid or a custom grid, in
order for the main media to appear, it needs to be
officially selected as one of the Media Sources:
When using a product-based grid, the image will be drawn from
the products itself. There are three possible ways an
image can be pulled from a product:
Only one of these sources are needed, so
you can choose what's most convenient for you. Each option
pulls the image from the individual product. And is related to appropriate image type in Magento. Where Featured Image in Magento Base Image and so on:
And for some reason if an image isn't assigned to a post in
one of the three ways listed above, when editing your grid,
in the "Source -> Media Source" section, you can also
set a "Default Image" that will be displayed in these
cases.
And finally, whenever you upload an image to Magento,
multiple versions of the image are created automatically
(thumbnail version, etc.). Because sometimes images
can be quite large, and you may not need such a large image
when it's placed inside an Essential Grid item, you can
use the "Set Image Source Type" option to choose which
version of your images you'd like to load into Essential
Grid.