If you are trying to align the images inside a post, I assume you have a class called "content" or "entry" or something like that.
You can use something like ...
Code:
#content img {
margin: 0;
max-width: 640px;
overflow: hidden;
}
#content .attachment img {
max-width: 900px;
}
#content .alignleft,
#content img.alignleft {
display: inline;
float: left;
margin-right: 24px;
margin-top: 4px;
}
#content .alignright,
#content img.alignright {
display: inline;
float: right;
margin-left: 24px;
margin-top: 4px;
}
#content .aligncenter,
#content img.aligncenter {
clear: both;
display: block;
margin-left: auto;
margin-right: auto;
}
#content img.alignleft,
#content img.alignright,
#content img.aligncenter {
margin-bottom: 12px;
}
Edit to fit your own needs.