Error 26 means that you might have a hardware issue. There are several ways that might help you out, such hard reset, refresh or upgrade iTunes, check out USB cable, use FORecovery iOS System Recovery and so on. Maybe you can find more introduction in the how to fix error during upgrading iPhone 6s article.
If you want to backup your photos before resetting, you can try to connect your iPhone via a USB cable. After detecting, just shut down iTunes and open "this computer" folder and import your photos to your computer. But this method might need to select trust on your iPhone. Anyway, just take a try.