How do I adjust opacity of a photo?

Hi

That is a meaningless concept in the photos app, since photos is not a layered editor (what are you expecting to see through your transparent photos?)

However, if you are using the photo in a document, then depending on the document editor, you may be able to make it transparent in the document editor. Pages can do this, for example.

You can make parts of a photo transparent in Preview, but it is all or nothing.

Photos has not been designed to handle transparency- We can import PNG files or GIFs, but if we try to edit them in Photos, the transparency will be lost.