(Tumblr) How to stop HTML changing itself?

I am noticing several days after completing my basic Tumblr page, that the post layout is now messed up because my original image tags have all been changed to something different than what I input originally (attached before and after pics). It breaks the center alignment of text among other things in my posts. I despise how HTML does this and only at a random point in the future (looked perfect when I previewed and hit apply on the day). I've no idea how to fix or workaround this?