Shrunken image despite being the same as previously working ok

Hi, I am really hoping there is some solution here. I usually put an image 1200x200 px at the top of my emails and it shows up fine. For the last week, when adding any image of this size, even ones which worked fine before, it shows up absolutely tiny and impossible to tell what it is - and even in the preview it is the same. Anyone know why this is happening and how I can get back to having a banner image on my emails. Thanks