You could create a custom text area field and then run campaigns which populate that field using the Lead Owner tokens combined with some HTML.
For Example:
Owner Signature as a custom text area field.
Run a campaign which filters by lead owner, and then in the flow have a Change Data Value flow step for Owner Signature, setting it to (for example):
<p><span style="font-size: 22px">Best Regards,</span><br>
<span style="font-family: trebuchet ms,geneva">{{lead.Lead Owner First Name}} {{lead.Lead Owner Last Name}}</span><br>
{{lead.Lead Owner Phone Number}}<br>
{{lead.Lead Owner Email Address}}</p>
<img src="http://www.yourdomain.com/SmilingImageOfSalesRep">
Then you could just reference {{lead.Owner Signature}} in the email. If you want to have different HTML for different lead owners, then just use choices or run a separate campaign. Keep in mind that this field is on the Lead/Contact object, not the SFDC user that Marketo looks up the Lead Owner information from, so it's important that it be up to date when you're sending out an email (so your leads don't see blank or incorrect signatures!).