max-width would be better, I agree.
I was going to note that just adding a style to
.postimage would affect the rest of the ZP forum, but then I remembered it’s not in use anyway. So it’s probably fine.
You end up with this left-alignment depending on window size, but when the image is forced to be 100% it starts to look out of focus.
An alternative would be to change it to a block image with auto margin:
That way you at least get a balanced centred placement:
It’s probably not really worth the effort though.