Inserting class=alignright attribute to img tags

I did a search through the forums for this. Ultimately I want to add more space around my images in a post. I know this is done by adding the attribute class=alignright (or alignleft) to the respective images. But I do not know how to do this. How can I get wordpress to automatically use the class=alignright attribute instead of the align=right attribute when I insert an image? Thanks