<edit>
From your page : http://em202020.tumblr.com/
You could do:
.posts a:first-of-type img,
.posts iframe {
margin-left:-80px;
}
Or if you like it better :
.posts a:first-of-type img {
margin-left:-80px;
}
.posts iframe {
margin-left:-20px;
}
.</edit>
first guess answer irrelevant. you can do :
.posts img:first-child {/* it will only select the first-child if it is an img tag */
margin-left:-20px;/* swallow that padding-left:20px; */
}