I am using a css class that steemit built into the system called "div class=pull-left" . When I do that they get resized smaller and they all flow down the page. Some images are larger than others and taller than the others. So they do not always line up.
You gave me an idea however. I should put them in a html table tag. That should let me control it better.
So far I have not found a way to inject css styles into steemit.
I will keep experimenting.