This cannot be done with a single Ditto call, but through the appropriate manipulation of the recursion it is possible to solve this task with a single clause that will meet all your requirements.
I assume that the mentioned documents ("GalleryOverview", "Portraits", "Portrait 01", ...) are all containers. Photos are located under "Portrait 01", "Landscape 01", "Black & White 01", etc. Right? If so, then the task might be solved with something of this kind:
[[Ditto? &parents=`10` &depth=`2` &display=`all` &filter=`isfolder,1,1` &tpl=`newest-photo`]]
The above will catch all containers under "GalleryOverview" for two levels in depth. Now the most important part, content of the "newest-photo" chunk:
[[Ditto? &parents=`[+id+]` &depth=`1` &display=`1` &filter=`isfolder,0,1` &tpl=`PhotoDoc` &sortBy=`createdon` &sortDir=`DESC`]]
This chunk pulls newest (latest) photo doc for the each container caught on the first level of Dittoâ€™s call. I have tested this idea and it is working.