check to see if a form feild is empty.

Ok I am creatying a script and in this script I allow for users to upload multiple pictures. Here is the script I have so far.

# DEALING WITH PIC_UPLOADS Part 1
if($HTTP_POST_FILES['picture2']){
     $picture2 = $HTTP_POST_FILES['picture2']['tmp_name'];
     $pic5 = getimagesize($picture2) or die("gis_failed");
     $ext = pic_allowed( $pic5[2] ) or list($submit_err_msg, $act) = array("pic_format_not_allowed", "signup");
     $ext = "." . strtolower($ext);
}else{
     $picture2 = $HTTP_POST_VARS['picture2'];
     $pic5 = getimagesize($picture2);
}
     
if($pic5){
       $pic2_width=$pic5[0]; $pic2_height=$pic5[1];
     }else{
       $submit_err_msg="pic_not_found"; $act="signup";
     }    
# DEALING WITH PIC_UPLOADS Part 2
if($HTTP_POST_FILES['picture2']){
     #GENERATING A UNIQUE FILENAME BASED ON THE TIME AND REMOTE IP - AND IF THAT ISN'T UNIQUE I DON'T KNOW WHAT IS
     $filename = "uploads/" . preg_replace("#\W#", "", $HTTP_POST_VARS['user']) ."-". md5(microtime() . $HTTP_SERVER_VARS['REMOTE_ADDR']) . $ext;
     copy($HTTP_POST_FILES['picture2']['tmp_name'], $root_path . $filename);
     @chmod($root_path . $filename, 0755);
     $picture2 = $filename; #For the database
}

I wanted make it where it will only get the image size if there is infact a image selected in the picture2 feild of the forum. See what I am getting is a error that says it can't get image size if I have not selected to upload a second picture. How can I make it to where the script above will only process if there is in fact a picture selected to upload. If there is no picture selected to upload then it will nt attempt to process this portion of the script.