Custom cards usually fall into two categories, the ones which represent existing cards but with an alternate artwork, and the ones which are completely new. Here some examples of these custom cards.
Since March 2019, DriveThru Cards offers a service to create VTES custom cards. (and even have them print for a fee). In addition, Black Chantry also has provided some design guidelines for creating crypt cards.
For a long time, the Spanish VTES website also had an easy-to-use tool which let you create custom cards (requires registration, but no fees attached), but unfortunately this service is no longer available.
Custom cards are typically used in casual games only, but are not allowed in official VTES tournaments.
Last Update: 17/Jul/2019.