The idea, as Eric Larson tells it, is that an Image Universe only exists so far as that other characters from other Image comics have counterparts in each book. So, the Invincible universe is separate from the Witchblade universe, but there's a Witchblade who also exists in the Invincible universe, who has probably had many adventures similar to the Witchblade from the Witchblade comic universe. But there may not be an Invincible in the proper Witchblade universe.
It sounds a little confusing when I type it down, but it boils down to: Continuity only needs to be title-specific. Otherwise, go wild.