writing posts and adding divs causes p tag in around div

Hi there,

When I add a div tag to a post it is automatically enclosed in a p tag. How do I stop that from happening?

I want ‘<div class=”img-dec-r”><img src=”picture.jpg” /></div>’

but get

‘< p > div class=”img-dec-r”><img src=”picture.jpg” /></div < / p >’

What do I do?

Many thanks