There isn't a way to upload your template to the Lead Gen Campaign gallery to use in the Email Sequence. And the process calls for showcasing a template, not a final cover that's already been built. Not only is it more labor-intensive, but it also eliminates the need to meet with the owner in the Pre-Production Meeting and delivery of their personalized covers.
Technically, of course, you can create a cover set for your lead ahead of time and use the Draft Link instead of the Lead Gen template for Cover Preview:
Can I Send the Lead Gen Campaign to a Cold Lead
What if My Lead Doesn't Have Social Media Pages, Can I Still Use Lead Gen Campaigns
Should I Re-Write the Emails for the Lead Gen Campaign
How would I know that the Lead saw the emails I sent them
How Soon Should I Follow up With a Lead