I think I understand what you want to do. I'm running Atahualpa 3.4 at the moment and what I did was go to the Theme Options -> Style IMAGES -> Images in Post and added
Width: 80%; (you could use 100%, it just looked wrong on my site)
To have the photo open at full size when clicked on just edit the photo, click on Advanced settings. There's a check box at the bottom with this option.
The only problem I've had with this option is that it affects my thumbnails on the index page as well. Still working on how to get them back down to 150px while keeping this style for the rest of the site.
Here's a page
from my site that works as I imagine you want yours to, although only the first image has been edited to open in a new window/tab.j
I found a better way. Assign a class to your image and then add a CSS insert:
You still have to do the other bit about telling it to open the image in a new window/tab when clicked. This way it doesn't mess around with all your other photos, just the ones you want displayed at the full width of the column.