XF 2.2 How replace xfrm button icon ? - icon="purchase"

[dondomainer]

Hello, how replace xfrm button icon - purchase icon ?

<xf:button href="{$resource.external_purchase_url}" class="button--cta" icon="purchase" target="_blank" rel="noopener">

this, of course, it doesn't work: <a href='{$resource.external_purchase_url}'> <img src = 'pic.jpg' /> </a>

Thanks.