You may have to scroll down to see the section "upload images to this gallery." Click on the browse button (see red circled area).
Navigate to where your photo is located. Once you see the path or image name in the box you can then click on the button that says "Upload Image."
Depending on your internet speed and the size of the file, this could take a minute or two or even longer.
When it's done, the screen will blank and refresh itself. The Upload Image area is now blank, and at the very top left of the screen is a message "Uploaded image: [image name]".


UPLOADING IMAGES AS A GROUP (SERIES):

There are two alternative ways of doing this. You can either:
(a) package your images into one *.zip file, then browse to it as above (red circled area); or
(b) use the yellow circled link (it's been moved to the left, just below the single upload field, since this screen shot was taken) to the "expanded upload mode" screen.
The latter has multiple boxes to list each image separately, so you don't need to Zip them first, and is quick and intuitive to use.
Click "full edit view" (it's a bit inconspicuous) in that screen when the upload has finished and the image fields have gone blank.