Sitefinity Blog Posting Image Margins
The <img> tag supports the following HTML attributes - hspace & vspace. However they work for both sides at the same time - so hspace will affect the left AND right spacing. Or you could always apply a custom attribute like this - style="margin-top:5px"
You can configure the editor to allow you to add custom classes to things...you could then apply CSS to style those elements however you want. Check out this post about how to enable the ApplyClass tool.