First post so go easy on me

The problem is when I apply a class to an image via the WYSIWYG is doesn't seem to render it. I've checked the html view and the class tag is there (class="image_left") it just doesn't seem to apply the style to the image (either in the WYSIWYG or the browser) - strange thing is when I apply the style directly (style="margin-right:10px;") to the image it works perfectly - I know I could do this manually to all the images but it would be a better solution for the end user to just apply the class.
Resaon for wanting this control is the h and v spaces don't display in Firefox but the margin does. Also the h and v apply margin to all sides which disrupts my content alignment.
Thanks in advance...