Preview won't show added picture

I created a slide show by prefixing my filenames with numbers so Preview would show them in the desired sequence. But after showing the slides once, I added a new slide later, with the correct number, so in Finder it shows up in the right sequence. But now when I "CMD-A" to select all, and "Open with...Preview" for my slide show, that one slide pops up in one window, and then another window with the first slide in the sequence, but never showing the "new" slide in sequence. How do I get the sequencing corrected? Context: Macbook Pro, Yosemite 10.10.1.