Is it true that it's Haram to take a picture without Hijab?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If what you mean by your question is the camera photos for a non-veiled woman (no Hijab) it is unlawful because the head is Awrah (a part of the body which must be covered from all marriageable men) according to the consensus of the Muslim scholars. But it is lawful, according to the scholars, for the woman to unveil her face for necessary photos like ID photos, passport, driving license and the like.
So, the photos should be limited to the minimum necessary.
As for having photos as souvenirs for future remembrance, or for hanging them on the wall, this is not allowed in Sharia. The evidence for this is the general interpretation of the Hadith narrated from the Prophet about photos and pictures. These photos and pictures become more prohibited if they are made of an unveiled woman.

Allah knows best.