Currently we use TextExtracts extension, but as discussed in this patch it would be better to use RESTBase.
We could probably reuse the $wgGENewcomerTasksRemoteApiUrl config variable to determine which language wiki to use with RESTBase.
Also, if we don't have a thumbnail from PageImages (which happens if e.g. $wgGENewcomerTasksRemoteApiUrl is used) then maybe we could consider using the thumbnail from RESTBase, even though it's not sized quite how we'd want it to be.