So... the way I understand it is Marksmanship in LoD has two separate conditions:
1) If your hero is an agility hero -- you receive +70 bonus agility (which in turn gives +70 bonus damage because any boost to your main stat causes a boost in damage)
2) If your hero is non-agility --- you receive +70 bonus agility AND +70 bonus dmg unless a hero is within a 400 AoE of your hero for 4 seconds, in which case the bonus damage temporarily disappears.
So the issue I want to report is that it seems like images (Phantasm, Mirror Images, Conjure Image, Disruption Images) fall under both conditions, receiving +140 damage from Marksmanship on agility heroes. I've noticed on several occasions when I pick Marksmanship + Phantasm on agility heroes that my images actually have more bonus damage that my base hero -- resulting in them doing more damage than my base hero.
I could be wrong, but it's something that usually is left unsaid or people do not notice.
Let me know what you think.