This is the default behaviour, due to the float being removed from normal flow. The floating image (and its margins) will overflow the containing paragraph by default.
Demo at: http://jsfiddle.net/m937Q/
The css I tested with:
p {
width: 300px;
background-color: wheat;
}
p img {
float: right;
margin-bottom: 10px;
}
Note:
If your div has a new block formatting context (will happen if it is floated or overflow: hidden; among other things) it will contain its floats and margins: http://jsfiddle.net/m937Q/2/