How to keep image sizes from changing?

We are working on a website using Vantage and Page Builder, where the page is divided into one side 66.7% and the other side 33.3%. We have estimated that the one-third column width is about 330 px and the two-third column width about 670 px. When we upload “full-size” images of the exact same depth (in this case 480 px), either Page Builder or WordPress alters the actual size so that the picture depths do not match.

You can see the problem at this example: http://lcitest.com/ic/plant.html

We have verified that both pictures are the same depth in pixels, and if you view the Text version in Page Builder, it confirms that. However once the page is published, the image sizes are altered, as we can see using the Develop > Page Source tool in Mac Safari.

How can we control this so both images are the same depth? We have a number of pages with this 2/3 – 1/3 layout where this is an issue.

Thanks