Click the 'Choose File(s)' button to select as many photos as you wish to upload. The photos are queued up and sent one at a time automatically.

You can select multiple files at a time in the file browsing window by holding down ctrl and clicking each photo, or you can choose a group by clicking the first, holding down shift, and clicking the last photo.

When you're done adding photos, wait for all of the photos to indicate they've completed, then click Next to add captions and re-order the photos.