Is raising one's hair to look like a boy's hair, drawing a mustache and goatee, wearing boys' clothes for a short period and then immediately taking them off, and taking pictures in that appearance without anyone seeing them except family members, considered an unlawful imitation of men?

The imitation of men by women is unlawful and a major sin. The Messenger of Allah, peace and blessings be upon him, cursed effeminate men and masculine women. Therefore, what the questioner did by imitating is indeed imitation, even if it was for a short time and no one knew about it. This is because the inclusiveness of the curse on imitators, as stated in the Hadith (definition), for every imitator, necessitates its application across all times and situations. Taking a picture in that posture indicates satisfaction with the imitation. The obligatory duty is to repent to Allah and cease this forbidden frivolity.