Usually, the person who actually takes the photo is the likely copyright owner, because copyright generally follows authorship. If the stranger makes creative choices such as framing, timing, composition, or settings, that strengthens the case that they are the author. If they are acting almost entirely under your direction, there may be an argument for joint authorship or that their contribution was not meaningfully creative, but that depends on the law of the country.

Under U.S. analysis often cited for this situation, authorship depends on who exercised the creative control over the final image.

Practically, this is almost never a real-world problem. A stranger is very unlikely to track the image, recognize it, or object to your personal online use. There is also a strong common-sense argument that, by agreeing to take the photo at your request and handing the camera back without conditions, they have at least implicitly allowed you to use that photo for the purpose it was taken.

So: technically, the stranger often owns the copyright, but for ordinary personal sharing, this is usually a non-issue. Laws vary by country.