put an if statement around the image tag to use a default static image if the recordset column is blank:
<?php if($row_SecurityAssistusers['UserImage'] !="") { ?>
<img src="../../login/profile_pics/<?php echo $row_SecurityAssistusers['UserImage']; ?>" class="img-thumbnail" width="200" height="200" alt="pfrofile">
<?php } else { ?>
<img src="../../login/profile_pics/default.jpg" class="img-thumbnail" width="200" height="200" alt="pfrofile">
<?php } ?>