I follow your code, but vs report an error said, Unrecognized type ‘FOnlineUserImageRetrievedDelegate’ - type must be a UCLASS, USTRUCT or UENUM.
When I remove the decoration(‘UFUNCTION(BlueprintCallable, Category = OnlineUser)’) above the function RetrieveThumbnail, the project can compile and no error.
I wonder how can you compile those code withour error