Hello,
You need to edit the postbit template you are using. (Either postbit or postbit_legacy).
Look for this code:
<if condition="$show['imageattachment']">
<fieldset class="fieldset">
<legend>$vbphrase[attached_images]</legend>
<div style="padding:$stylevar[formspacer]px">
$post[imageattachments]
</div>
</fieldset>
</if>
Change it to:
<if condition="$show['imageattachment']">
<div style="padding:$stylevar[formspacer]px">
$post[imageattachments]
</div></if>
They will no longer be displayed in the "box". (the fieldset.)