2.) After creating the new folder, enter this new folder just by clicking on it.
By clicking on the green button "Upload" you can upload the images you want to show in your photo gallery either from your PC or from Dropbox directly into this folder. You can also move images you uploaded in the past into this folder (and skip the upload here).
Now you can create your gallery:
3.) In the menu, click on "Addons" => "Galleries" => "Create a new gallery".
4.) Select the image folder which you want to use for this gallery (the one you created before).
5.) Choose the gallery type.
6.) Enter a title for your gallery.
7.) Decide whether the new gallery shall be included immediately in the visible menu of your website and finally confirm by clicking on the "Submit" button.