[TheLastSuperman]

Note: Overkill if only wanting avatar centered however... You could center all of it, some prefer all of the userinfo centered (within reason) here is a snippet of code that will do so (most of which I copied from a post by Lynne loooooong ago other additions over time).

/* center avatar, username, title, rank, userinfo_extra, postbit_reputation, imlinks */
.postbitlegacy .userinfo .username_container, .postbitlegacy .userinfo .rank, .postbitlegacy .userinfo .usertitle .imlinks {
	text-align:center;
}
.postbitlegacy .userinfo .postuseravatar, .postbitlegacy .userinfo .postuseravatar img {
	float:none;
	text-align:center;
}
.postbitlegacy .userinfo .usertitle {
	display: block;
	float:none;
	text-align:center;
	clear: both;
}
.postbitlegacy .userinfo .userinfo_extra {
	text-align:center;
	clear: both;
}
.postbit_reputation {
	float:none;
	text-align:center;
}
.postbitlegacy .imlinks  {
	float:none;
	text-align:center;
}
.postbitlegacy .after_content {
	clear:right !important;
}

I can't recall if the items listed like location would look wonky, if so you could try also adding:

.postbitlegacy .userinfo .userinfo_extra dl {
	text-align:center;
}

.postbitlegacy .userinfo .userinfo_extra dt {
	float:left;
	text-align:right;
}

.postbitlegacy .userinfo .userinfo_extra dd {
	float:left;
	text-align:center;
}

^ I had the bottom code commented out using /* .cssdefinition {code etc} */ so can't recall why in my one test style I did that, might not need to add the bottom code but there for reference.