How do you make a single slide in Keynote portrait for a vertical photo, without changing whole slideshow?

It's not possible to change to a vertical format for a single slide, or a set number of slides in Keynote.

A vertical format in Keynote only works if you are using a vertical display, such as the large vertical displays seen in shopping centres.

Lylamae wrote:

How do I only change a single slide to portrait?

I don't think that is possible in Keynote itself.

However, you could File > Export to > PDF...

Then open the PDF in Preview, select the slides that need rotating (by shift-clicking thumbnails on the left), then Tools > Rotate Left or Tools > Rotate Right and save.

SG

Hi there Lylamae,

Thanks for taking the time to connect with us here in Apple Support Communities! We understand you're asking how to change the orientation of some photos in Keynote on your Mac. We're glad you're here, and we can provide some guidance.

"Note: You can’t flip a table, chart, or image gallery.

1. Click the object to select it, or select multiple objects.
2. In the Format  sidebar, click the Arrange tab.
3. Click a Flip button (near the Rotate wheel) to flip the object vertically or horizontally."

Have a look at this link to learn more:

Resize, rotate, and flip objects in Keynote on Mac

We hope this information is helpful. Don’t hesitate to let us know if you have any more questions.

Cheers.