Serious issues with final email not looking like it did when creating it

So this issue keeps coming up, but the last email campaign I sent out to our entire contact list really made me mad and made our company look like we don't know what we're doing..

So I've always noticed slight differences in how a campaign email will look when I'm building it verses what it looks like when sent out. Normally small things like a header I had on one line turns to 2 lines, or slight differences in how the font itself will look.. But in this last email I had a body of text beside an image of our website homepage and everything looked fine in the builder section, it looked fine in the preview and when I sent a test email to myself like I always do.. So after I completed it and sent to our contacts, I get an immediate email from my boss asking what was wrong with the latest email campaign.. I go to look at it and the image I had placed in took up the width of the entire email and my body of text was on top of it and completely illegible. It's a complete mess and failure of a really important email campaign announcing our new website. So that's just great. I'm attaching pictures of the preview image and what the actual email looked like when sent out.