Creating a slideshow in photo
How do you insert a text only slide (title page) into a slideshow in the Photo app?
iMac Line (2012 and Later)
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You can only add a title to an existing slide. If you want a plain background for the text, insert a photo of a plain background as a slide.
At the right end of the strip with the slides is a "+". Select the plain slide, then click the +, then click Add Text. Select the text in the text box, then type the text you want.
https://support.apple.com/guide/photos/create-slideshows-phtae8c6d40/mac