Any time you’re dealing with images from disparate sources and with differing compositions you’ll almost certainly need to do some manual adjustment of the images after merging. It’s just the nature of the work. Add just a couple of simple steps at the beginning, and you can save yourself minutes, hours, even days of individually fitting images by automatically fitting them during merge.
Excerpted from Pariah Burke’s comprehensive Pluralsight video course, InDesign CC: Data Merge & Variable Data Printing.