This is what I have done which might point you in the right direction:
1) It took me forever to get the right path to extend the template from vendor\magento\module-grouped-product\view\frontend\templates\product\view\type\grouped.phtml
to my theme directory app\design\frontend\Vendorname\Themename\Magento_GroupedProduct\templates\product\view\type\grouped.html
2) I added the line <?php $productImage = $block->getImage($_item, $image); echo $productImage->toHtml(); ?>
3) the trick now is to figure out how to define what goes in $image
- i have created my own definition in my theme app\design\frontend\Vendorname\Themename\etc\view.xml
<?xml version="1.0"?>
<view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
<media>
<images module="Magento_Catalog">
<image id="grouped_list_product_thumbnail" type="thumbnail">
<width>75</width>
<height>90</height>
</image>
</images>
</media>
</view>
So for me, $image = "grouped_list_product_thumbnail";
This is working for me although it is showing the placeholder and not the thumbnail - it seems my test migration did not copy the thumbnail linkage properly
This might help you solve yours though :)