Here are two ways to style images using Markdown
If you'd like to use classes, HTML is completely valid in Markdown:
<span class="right">

</span>
In CSS you can then target:
.right img { float: right; }
Easy!
For something a little cleaner, you could also take advantage of Markdown syntax as a shortcut. For example, here's how you could get 3 different image styles using just Markdown and CSS:

**
****
This would render as:
<img src="/assets/image.jpg">
<em><img src="/assets/image.jpg"></em>
<strong><img src="/assets/image.jpg"></strong>
Then you could target in CSS using:
img { /* default */ }
em > img { float: right; }
strong > img { width: 100%; }