The reason some avatars don't show up is because your user didn't choose one:
The Fix:
Find in memberlist_resultsbit:
PHP Code:
<td class="alt1Active" align="$stylevar[left]" width="10%">
<img src="$avatarurl" />
</td></if>
Replace With:
PHP Code:
<td class="alt1Active" align="$stylevar[left]" width="10%">
<if condition="$avatarurl">
<img src="$avatarurl" />
</if>
</td></if>
That's it... How ever if you want an image saying this user has not choosen an avatar:
Download:

"Image Taken From Default Avatar Mod"
Find in memberlist_resultsbit:
PHP Code:
<td class="alt1Active" align="$stylevar[left]" width="10%">
<img src="$avatarurl" />
</td></if>
Replace With:
PHP Code:
<td class="alt1Active" align="$stylevar[left]" width="10%">
<if condition="$avatarurl">
<img src="$avatarurl" />
<else />
<img src="$stylevar[imgdir_misc]/noavatar.gif" />
</if>
</td></if>