If you use <center></center> around an image, it should be centered within the enclosing div. However, it looks like Steemit strips out image size tags, which means we can't directly resize the image?
Code: <center><img src="https://developer.chrome.com/extensions/examples/api/extension/isAllowedAccess/sample-128.png" width="10" height="10"/></center>
So, the method using the Steemit-provided CSS classes pull-left and pull-right seem like the only things available. I tried resizing the div (also rejected) and a style (also removed) which is pretty standard for a CMS that has to handle user-provided content without barfing or causing security problems.
So, I don't think there is going to be a nice way to get the image size you want after the fact, it would have to be generated in the correct size right away. You might try using an SVG instead of a PNG if you want to be able to rescale, but I don't know if Steemit's image hosting supports that (it might automatically translate to a PNG) and as mentioned above, Steemit seems to strip out any formatting tags that would help.
Thanks a lot for your reply, I'll try changing the image format as you suggested. If that doesn't work the only thing I've got left is trying to rescale the PNGs using photoshop. I don't know why but when rescaling them with an online app I always got fuzzy images.
I was intending to do an article on classical mechanics, mainly Noether's theorem and it's use to derive Newton's third law and conservation of energy, but since we can't use MathJax and this doesn't work I think I'm going to give up.