Welcome to iMore!
Here is a guide that will assist you in doing just what you want. Then put the photos on a flash drive once they are on your Mac.
On Your Mac:
- Go to the photos app.
- If you already have pictures in the app that you don't want to upload, move your library file and create a new one. Set the new one as your system photo library by going to Preferences->General->"Use as System Photo Library".
- Make sure that Preferences->iCloud->"Download Originals to this Mac" is checked.
- Check Preferences->iCloud->"iCloud Photo Library".
- Now all your iCloud photos will download.