To add product images to the quote document, first follow the instructions for customizing the quote document. Product images can be added in the {block name="document_index_table_each" append} block, for example with the following code:
{if $position.articleDetail->getArticle()->getImages()->first()}
<img src="{$position.articleDetail ->getArticle()->getImages() ->first() ->getMedia() ->getPath()}" width="10%" />{/if}