do you mean the logo of your company in the head of your website, to the left of the main menu?

Customise >> Main Layout >> Show Logo should work..

anyway you can overwrite the default template, below the section you should edit in website/views/website_templates.xml

<template id="layout_logo_show" inherit_id="website.layout" customize_show="True" name="Show Logo">
    <xpath expr="//header//a[@class='navbar-brand']" position="replace">
        <a href="/" class="navbar-brand logo">
            <img src="/logo.png" t-att-alt="'Logo of %s' % res_company.name" t-att-title="res_company.name"/>
        </a>
    </xpath>
</template>

remember to put the customization on a module so you don't loose everything when you update the code