First of all, forgive me for asking the blond question but, you did change "MYCUSTOMERID" and replaced it with your GUID key right?
I've tried two options, first a generic content block, switched to HTML tab and pasted an entire snippet in it. (left example on the screenshot)
In both cases the seal is shown. If yours still doesn't show up, you can check the network tab to see if seal.js and secure90x72.gif are being loaded from verify.authorize.net. If they are, you might want to view the source and see if there are some CSS issues causing the not showing.