First, you will need to install iFuse:

sudo apt install iFuse

Then, it is really simple:
1. Connect your iPhone with USB to your laptop.
2. Unlock your iPhone.
2. Run idevicepair pair.
3. The phone will ask if you want to trust this computer, say yes.
3. Open the folder Dolphin/file browser. For me, I get a “open camera” notification that leads me to the picture folder in the file browser. You can also try camera://<id given by idevicepair> or afc://<id given by idevicepair>.
4. Copy the files!

When I did this, I had to reset the connection with idevicepair a few times, but other than that it worked very well.

If this does not work for you, try this very thorough guide by Dedoimedo/Igor Ljubuncic: How to access iPhone (6s) with iOS 11 in Linux