If you want a new variant type to be controlled by the variant types linked to a publishing destination itself (on the Variant types tab) here's the way how you can access those values and include them into the variant workflow activity.

This will work for 'image' objects only.

<Variant 

 x:Name="createVariants" 

         ObjectIds="{wf:ActivityBind Replicator, Path=ObjectIdsOut}" 

         Data="{wf:ActivityBind LoadImage, Path=DataOut}" 

         CreateAlways="true">

   <!-- log the errors in variant creation -->

   <Log 

                 x:Name="logVariantCreate" 

                 XmlMessages="{wf:ActivityBind createVariants, Path=XmlOut}" 

 />