Alright, after seeing this blog post https://odedrabhavesh.blogspot.com.co/2015/01/how-to-replace-kanban-image-in-odoo.html I got the solution. Although this example from the blog post works for Odoo8:

Code: 

<record model="ir.ui.view" id="res_partner_kanban_view_extened">
    <field name="name">res.partner.kanban.view.extened</field>
    <field name="inherit_id" ref="base.res_partner_kanban_view"/>
    <field name="model">res.partner</field>
    <field name="arch" type="xml">
        <xpath expr='//kanban/templates/t/div/a/t/t[@t-if="record.is_company.raw_value === true"]' position="replace">
        <!-- check condition weather IS A COMPANY is checked or not -->
            <t t-if="record.is_company.raw_value === true">
                <img  src="custom_patht_of_image" class="oe_kanban_image"/>
            </t>
        </xpath>
    </field>
</record>

I had to modify it as Odoo v9 no more supports named attributes of Xpath (like @class='page', etc.).

So this was my solution, working with Indexes instead:

<record model="ir.ui.view" id="res_partner_kanban_view_extened">
            <field name="name">res.partner.kanban.view.extened</field>
            <field name="inherit_id" ref="base.res_partner_kanban_view"/>
            <field name="model">res.partner</field>
            <field name="arch" type="xml">
                <xpath expr='//kanban/templates/t/div/div[3]/ul' position="before">
                    <field name="myvar" />
                    <div t-if="!record.myvar.raw_value"
                         style="margin: 0px; padding:3px 3px 10px 3px; color:#F77171; background-color: #FFF3F3;">
                    </div>
                </xpath>
            </field>
        </record>

Hope this helps someone :-)