Uploading files doesn't always work

[garydt]

I have a website that lets users upload jpeg files. Some users upload fine but when others try they get a page full of warnings and lose the site. What would cause this?

 

Also, now when users are logging in they are logging into other users accounts but it was working fine 2 days ago. I haven't changed the script so why is this happening?

Thanks.

[garydt]

shall I post the uploading file script?

[garydt]

I've worked out the log in problem but still can't why some users are able to upload photos but others can't. Heres the upload script-

 <form name="form1" id="Upload" action="<?php echo $editFormAction; ?>" enctype="multipart/form-data" method="POST"> 
        <p> 
            <input type="hidden" name="MAX_FILE_SIZE" value="<?php echo $max_file_size ?>"> 
        </p> 
        <h3> 
          <input name="file" type="file" id="file" onchange="MM_showHideLayers('Layer1','','hide')" />
          <span class="style1">for your photo</span> </h3>
        <p>
          <label>
          <input name="sent" type="hidden" id="sent" value="1" />
          </label>
        </p>
        <input type="hidden" name="MM_insert" value="form1">
</form>

 

$editFormAction = '';
$max_file_size = 1024*1024*1024; //1MB
if(isset($_POST['sent'])){
unset($submit);
if (isset($_POST['sent'])) $submit = true;
else die('Error: No file got sent');

 

$uploadsDirectory = './uploads/'; 

//Upload image
$uploadFilename = $uploadsDirectory.$fieldname['name'];

// now let's move the file to its final location and allocate the new filename to it 
move_uploaded_file($fieldname['tmp_name'], $uploadFilename);

$user = ($_SESSION['MM_Username']);
$a = rand(0,9999);
resizeimage($fieldname['name']);
rename("thumbnails/".$fieldname['name'], "thumbnails/".$user.$a.$fieldname['name']);
$smallimage = './thumbnails/'.$user.$a.$_FILES['file']['name'];

resizelarger($fieldname['name']);
rename("smallpics/".$fieldname['name'], "smallpics/".$user.$a.$fieldname['name']);
$smallpics = './smallpics/'.$user.$a.$_FILES['file']['name'];

unlink($uploadFilename);

Any ideas?