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. First of all, enable the plugin in the Settings → Configure digiKam → Kipi Plugins section. Configure your device to use the MTP protocol for use with an USB connection, and connect the device to your machine running digiKam. Select the desired photos and choose Export → Export to remote computer.
Press the Select target location button, choose Network → MTP Devices, and you should see the connected device. Navigate to the target folder on the device, and press OK. Hit the Start export button to transfer the selected photos.