You have to open the 3uTools on your iDevice when you import photos from PC to your iPhone’s camera roll.
If you can’t open the 3uTools on the iPhone running iOS9 version or above, you have to set “Trust” on your iDevice.
When you open the 3uTools on your iPhone, it will show you a prompt that “Mood diary” would like to Access Your Photos”. Then At this moment, please click on the button “Ok”, then after the photos will be automatically imported from PC to your iPhone or iPad.
If the PC doesn’t start to import photos after you open the 3uTools, please double click on “Home” button of your iDevice, and then open the 3uTools again.
Keep a good connection in between your iDevice and PC. Please don’t unplug the USB cable when you import photos to your iDevice and just wait until this process is finished.