Many blocks will have alternate layouts; make sure you "cycle through block layouts" to see if one is right justified or left justified in addition to the centered block:
In ONTRAmail templates the image block is a full width image with no alternate layouts (since a full width image is always the full width regardless of justification). Use an alternate block such as the Product Details block that uses smaller images and has centered, left and right justified designs. Then, toggle off the elements you don't need:
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