max width of posts

I added an image to my blog that is larger than my post area, i.e. it overlaps into the sidebars. Obviously I would prefer that it doesn’t. I know I can use the <img> tag to shrink it (<img=”myphoto.jpg” width=”20″ height=”20″). However, in this case I would prefer to shrink the image without using the <img> tag. I think that means I have to play with the site’s code somehow, but I’m not sure.

So I guess my question is, is it possible to set the maximum width of your posts? Or is there another/easier way to do this?