Couple of things you can do, the steps are basically the same for Android and Iphone
1. Upload the photos from the USB stick to the Google Photos or Apple Icloud web-sites and they will sync to your phone over wifi. You can also upload the photos to cloud apps like DropBox, Google Drive, Box, etc. You may need to manually install those apps on your phone though.
2. Connect your phone to your PC using a USB cable.Your PC should recognize it as a second USB drive. You can then copy files from your USB stick to your phone. You may want to put a folder on your phone called 'Photos', etc. You can also copy the photos from the USB stick to the 'DCIM" folder that's on your phone.