To style assets in Siteleaf you can use asset metadata. For example, you can create a meta field called css_class
on your asset.
Now in your theme code you can write:
{% for asset in assets %}
<img src="{{asset.url}}" class="{{asset.meta.css_class}}">
{% endfor %}
For more on asset metadata see: http://www.siteleaf.com/blog/new-asset-meta-and-quick-post-edit/
If you need to style images within your Markdown body, see: https://gist.github.com/sskylar/2faedc4ac82aef2631b9