how can i transfer pictures from my macbook to a flash drive

Open Photos app, select the images you want to save to your flash drive, click on File> Export, from the sub menu select either Export xx Photos or Export unmodified originals, from the next drop down select the options for how to save the files etc, Then press Export, the next dropdown you can select where to save your images, your flash drive should be in your sidebar ( if you have selected that option in Finder> Preferences ) or select a folder or create a New Folder, press Export. If saved to folder then just drag that folder to you flash drive.