Short gravatar question

Hello, I am using this to get the users gravatar:
<?php print $post->user_avatar; ?>
I have wrapped this inside a style forcing the gravatar to be resized to height and width 50px, the width is OK, but the gravatars image won’t downscale to 50 px. Why?
Here is the css I used to wrap the gravatar:

.lentryavatar {
width:50px;
height:50px;
background-color:#ffffff;
float:left;
padding-bottom:35px;

}

Any suggestions?