What is the true origin of the spirit-like image captured by a motion-sensitive hunting tree camera the owner feels was an angel?

This was an artifact created artificially for the purpose of misleading viewers and is not authentic. Keep in mind that a divine intervention cannot be offered unless it is invited. A performance for an absent audience would be ill-advised in keeping with our rules to engage only the believers who are in alignment, and to not offer proof of the divine to the nonbeliever. Most such photos, when they occur, are earthbound spirits not angelic beings. So that is the most common reason for such displays.