You just need to add below line in order line or invoice line table,

<img t-if="l.product_id.image" t-att-src="'data:image/png;base64,%s' % l.product_id.image" style="width:90px; height:90px;"/>

where, l is,

<t t-foreach="o.invoice_line" t-as="l"> or <t t-foreach="o.order_line" t-as="l">

For example,

if you want to add product title and image using xml than,

<table class="table table-bordered">

    <thead>

    <tr>

        <th><strong>Product Title</strong></th>

        <th><strong>Image</strong></th>

    </tr>

    </thead>

    <tbody>

    <tr t-foreach="o.invoice_line" t-as="l">

        <td>span t-field="l.name"/</td>

        <td><img t-if="l.product_id.image" t-att-src="'data:image/png;base64,%s' % l.product_id.image" style="width:90px; height:90px;"/></td>

    </tr>

    </tbody>

</table>

Regards,