mareep
There are two things, one is to transfer the photos, and the second one is to display them afterwards in a gallery.
The transfer should be simple enough, just select your photos and insert them into a folder of your choosing in your phone. If you see that the photos are being copied, than good, you have a connection, if you don't see it, try different port on your computer or/ and other cable. Once you connect your phone to a computer, pull down notification bar and choose File transfer instead of charging. Than you can transfer your files.
Be aware that android has usually a folder DCIM for your camera and there is always a folder Pictures. Nevertheless, you can choose whatever folder you want, you can even create new one, but to display your photos, your photo gallery need to have a permission to access this folder, otherwise you would see your photos in Files, but your gallery wouldn't be able to display them. Find your gallery app in Apps and set Storage scopes, so that the gallery display your photos. Select all folders that you want your app has the ability to display pictures from and that's it. You should've had your photos in your phone and you should've seen them in your gallery.