While looking at the site, choose 'Add a page' from the top navigation.

Choose the 'GALLERY PAGE' option and click next.

Choose the location of the page, give the page a title, description and keywords. 

You can then enter some introductory text in the content editor, then click 'save changes'

Next, navigate to the gallery page you have created and choose 'Manage Photos' from the top navigation.

From here you can browse for the desired image, then press 'upload image' to upload the image to the gallery.
Multiple images can be uploaded at once by clicking 'add another row'.