A childhood photograph can show happiness accurately without proving happiness was constant.

A happy photograph is accurate about the moment it captured. It does not testify to the months around it, and it cannot be used either to prove that a childhood was fine or to prove that the happiness was false. People smiling in a picture were genuinely smiling, and that is all the picture knows.

If images from your past sit awkwardly against your memory of it, you do not have to resolve the contradiction. Both can be true. A difficult period can contain real good days, and a good day does not settle anything about the period.