How do you pick a custom avatar?

You have to be a donor (of at least Bronze or higher).

Here's where you go to donate:
http://www.hcrealms.com/forum/payments.php

Additionally, different levels of donor receive different types of custom avatar (varying in file size to animation).

What does it take to get a blood donor status ? Article submissions, moderatorship, renoun ?

It comforts me that you are 8,000 posts deep and still don't know this...

Cause I've been wondering since I joined and felt like I was missing something.

Seconded. I just assumed it was part of the donor perks.

Need to be a moderator.
If you write articles you get the "contributor" tag

To expand on what DemonRS said; All "staff" (Admins, Moderators, and myself) have access to the "Blood Donor" badge, which represents the
"blood, sweat, and tears" we've put towards the community. The Contributor tags is for any official content contributor to the site; including articles, podcasts, etc. In addition to that tag; they are awarded donor levels for their articles.

We'll be mixing things up in the future because there are a lot of good writers out there, that can't keep to a weekly schedule but we'd still like to post their work on the front page.

thanks for the info!