@jinsan The alignment of the gravatar is set by adding style features to the img tag. So, you could have
<img src="<?php post_gravatar ?>" style="float:left; and any other style attributes">
You could also insert a style class and control your gravatar behavior from your style.css. So, you could have
<img class="postgravatar" src="<?php post_gravatar ?>">
And you could do the same for the comment gravatar. If you wanted less work and you wanted your gravatars to behave the same way on posts and comments, then you could have just one class for “gravatar” and style that in style.css. This would also help trim your stylesheet size. Personally, I think the reason you couldn’t tell what was happening with mine is because I haven’t styled gravatars, but I styled any image that is an anchor (img a { in css) to float left. Who knows, I may need to change that in the future, but for now it is working just fine.
Let me know if I can be of any more help.
Godspeed,