CanonsJohn
Photos for German aircraft have been explained many times on the forum. The + symbol acts as a divider in the airliners.net photo search. Instead of the system searching airliners net for registration '10+23' It searches for a registration that photo whose data field contains the number 10 and seperately the number 23.
If you go to airliners.net and type 10+23 into the quick search, you will see what I mean.
It is better to use database explorer to remove the iron cross as all German mil aircraft are loaded onto airliners.net without it. This aircraft photo will be found if you change the registration to '1023'