Upload the logo file in a File Area, get a permalink and use this code inside the footer in the Package.
<span class="copyright">Powered by
<a href="LINK-to-DOMAIN_here" target="_blank">
<img src="permalink_here" title="Company name" alt="Company name">
</a>
</span>
NOTE: This snippet is provided as is, with no express or implied warranty for accuracy or accessibility.