Provided that you use KDE, you can push photos from digiKam directly to a mobile device that supports the MTP protocol (e.g., most Android devices) using the KioImportExport Kipi plugin.
This is a companion discussion topic for the original entry at https://www.digikam.org/news/2015-01-23_digikam_quick_tip_transfer_photos_from_digikam_directly_to_a_mobile_device/