Members, Events, Groups, DKP Dungeons and other sections of your site can all have Avatars set for them.  And, when they do not have an Avatar set, Shivtr will show a default image instead.

If you want to set your own default, this can be done with CSS!  Here's an example for the members section:
 

/* Hide the default avatars */
#members_index img[src="http://cloudfront-assets.shivtr.com/s3/missing/member_avatar_small.png"],
#members_show img[src="http://cloudfront-assets.shivtr.com/s3/missing/member_avatar.png"] {
  display: none !important; 
}

/* Add your default avatar to members list */
#members_index .member_avatar {
  background-image: url(small-image.jpg);
  background-size: contain;
  width: 50px;
  height: 50px;
}

/* Add your default avatar to members show */
#members_show .member_avatar {
  background-image: url(image.jpg);
  background-size: contain;
  width: 96px;
  height: 96px;
}

And it should work, at least in most modern web browsers (chrome, ff 4+, etc.).  

You can add this to more sections of your site as well, though this is a little advanced as the node names will be a little different for each section, but with Google Chrome you can right click the page and go to Inspect Element and find these in most cases without too much trouble.

If you need to know the avatar sizes we use throughout the site they are:

Large: 96x96
Small: 50x50
Tiny: 20x20
Icon: 13x13
Comments: 35x35

Happy Customizing =)