For an even more professional image, replace yourname.pelli.io with your own domain.
Key points 
Available with the Ultra plan, in beta mode only
You link your domain via a CNAME record at your host
The connection is verified automatically (pending β in progress β active)
Connect your domainIn your settings, under the Custom domain section, enter the domain (or subdomain, e.g. photos.mystudio.com) you wish to use.
Add the DNS recordAt your domain host, add the CNAME record indicated by Pelli (CNAME type, with the value provided). This is the step that links your domain to your page.
VerificationPelli verifies the connection automatically. The status goes from pending to verification in progress, then active once the domain is linked. DNS propagation may take a little time on the host's side.