I am completely having the same issue as Gail and being my perfectionistic self, not willing to just do a text align or use a rich text module to center the image.
I somehow didn’t understand Gail and I were having the same issue when I first read this post, but it was so , so wonderful to read tonight. I also thought that obviously there must be a simple fix to not have an image be left-aligned.
I had also tried adding inline CSS to display the image as a block which doesn’t appear to work. I also tried to add it to the attached stylesheet as a custom class and no dice. I’ve tried margin: auto, margin: 0 auto (as you suggested above), margin-left: auto; margin-right; auto. Nothing works. I still feel somewhat crazy because obviously I cannot be the only person at Hubspot who wanted to center their image.
Anyways, if you take a look at my thread let me know if you have any additional thoughts. I posted a link to my page there as well.