You can choose your own avatar if you are a Gold Member. Just click on any of your pictures and click the Set As Avatar button underneath it. You can change your avatar this way as often as you like. Avatars showing in the Gold Members section tend to get more comments as they are visible more often across the site, so you can use this to promote new works, or perhaps older works that you would like more feedback on.
If you are a Gold Member, you can also sort all your pictures to show in the order you prefer on your artist page.
If you are not a Gold Member, your main picture is automatically set to be your current most popular one based on recent comments, and changes accordingly. This is to provide a variety of images over time for viewers, and so that the community can decide which pictures are shown, as this main picture is also shown on all your comments, and can appear on every page while you are online.
New pictures tend to become your avatar fairly quickly once they have received a few comments.
To select the picture that will be the first one on your artist page, you can do that using the Sort button on your artist page.