Missing images
All references to subdomains of the domain entered in the temporary link creation form will be updated from "sub.example.com" to the format "sub-alias.skipdns.link".
However, keep in mind that if you are using the tool to verify a migration, subdomains will not be loaded from the IP address defined when creating the temporary link: requests for subdomains other than "www-alias.skipdns.link" are proxied by our servers, but forwarded to the IP addresses they are pointed to (following DNS resolution).
You may see missing images or a 'broken' website layout, especially if your current hosting provider where these subdomains are hosted is offline or in case the source server disallow image hotlinking from unknown domains (they see your temporary skipdns subdomain as the origin on requests).
If your site doesn't look the way you expected, contact support at your new hosting provider - where you pointed the temporary link.
They will be able to detect problems due to the use of offline external subdomains, any incompatibility in their infrastructure with your website requirements, or just a matter of tweaking some settings (like selecting the appropriate version of PHP for your hosting account).