I put the first one three fingers down like a men's shirt but with the scooped neckline that places the center of the image between the breasts. It doesn't look bad. Should I leave it there or move it up?
I have found, that it really depends on the woman it's being sold to
The super flirtatious ones will like it right on them boobages, those going for a more professional look will want it above.
Rick, who is your target market? Figure that out and go to the mall where that crowd shops in the brick and mortar store, be it A&F, Gap, BR, Macy's. Follow your market to their physical retail location and measure where they put images. You have to establish a standard somehow, this one seems like a plausible way to me. Anyway, go back to having "boy fun". Thought I'd toss another actual answer in here before it got too "locker room'ish"!!
This is a custom order and while not flirtatious, it doesn't need to be conservative. Actually knowing the group they range quite a bit physically so there is probably no one good place. I was just wondering if there were a rule of thumb for women's tanks like tees.
My first thought would be that the image should be located on the "person" in the same place regardless of the shirt. That being said, I would adjust the distance from the front colar to account for the lower "scooped neck" of the tank and use that to remain consistent on all the shirts / tanks.
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I normally check to see how my images line up with the bottom of the armhole. Xsm-Small needs to be a bit different than a 2X-4X for the image to sit properly on the shirt.
Men like the image a little lower than women, generally speaking.
On a ladies' scoop neck t-shirt or tank top, I put the image a bit closer to the neckline. Again, I look how the image lines up with the armhole.
Where to position image on women's tanks? 
