They are very mystical as the man was himself. The images are interesting interpretations of what he was trying to say and help to explain his true meaning. This goes back to the whole idea that he wanted to be a painte first and that visual art was his ideal outlet. Whether he suceeded in any measure is something else again. The Lamb and the Tyger are two sides of the same individual, and each is neccesary to facilitate the other's existence.
Nothing is good or evil unless mankind makes it so, to coin a phrase.
He wrote with paint almost with the way he described things. His prophetic poems and The Marriage of Heaven and Hell overflow with symbolism and allusions that were extremely meaningful. He didn't live by anyone else's rules and deeple concerned with relaying a very personal message of truth, salvation, innocence, decadence, history, and criticism to the people of his age and ours in the only ways he knew possible. His work is not as complex as the later Romantics - even Wordsworth develops Blake's ideas better than the man himself seems to. However, his singsong rhymes convey a childlike sense of simplicity and innnocence that, I suppose, allows him to explore the metaphysical world in a more comprehensible manner.