I am preparing an updated CV for application for postdoc positions. Is it necessary to put a photo on a CV for postdoc positions?
CV formats vary widely between countries. In the US, Canada, the UK, Ireland, the Netherlands* and Sweden CVs should not include photos, as these could reveal personal information (on race, age, perhaps religion) that employers are not allowed to use under local laws (source).
In many European countries, except those mentioned above, it used to be or still is expected to include a photo on your CV. However, in internationalized areas like academia, norms are converging toward the Anglo-American/Nordic model. German universities, for example, no longer expect photos on CVs, but it's not wrong to have them. I didn't put a photo on my application and got a postdoc in Austria.
I'm not sure about China and Japan.
*Another source specifically on the Netherlands is less strict: Employers are reported as being indifferent, unless it's a high-ranking (e.g. international?) position, in which case including a photo is seen as "bad form".