Problem creating thumbnails

I've created a gallery section of my web site. The index page looks to see if there are any sub-directories and, if there are, looks inside for Jpegs. It then creates thumbnails on the fly from the jpegs and displays then in tables, suitably linked. Here's the problem. The whole shebang works fine on my local system, whether running under WinXP or Linux. My hosting provider says that all necessary GD libraries are enabled on the host system, so in theory it should work. But it doesn't. What happens is that the browser throws up image placeholders (because it comes across the <IMG SRC... tag) but then these disappear as whatever is coming back from my thumbnail routine is obviously empty. So before I start whining at my hosting company, I'd appreciate any help people could give. Here's the relevant code - I've included quite a lot of it, so sorry about the long post. The index.php page includes a function that builds the table incorporating the thumbnails. This has been passed the parameter $sourcedir which contains the name of a subdirectory under the current one, plus "/". The function looks at the files in this subdir and identifies Jpeg files using getimagesize(). It puts the filename into $file. The next important line is: echo "<IMG SRC='thumbnail.php?img=".$sourcedir.$file."'>"; When I view the output HTML (on both local and hosted versions), the output from this line is (eg): <img SRC='thumbnail.php?img=new/cherries-01.JPG'> In the same directory as the index.php file is thumbnail.php. This takes the jpeg image, makes a new smaller version and streams it to index.php. The code for this is: <?php header("Content-type: image/jpeg"); $sourcefile = stripslashes($_GET['img']); // get filename passed via GET $maxdim = 100 ; // set the maximum dimension for the thumnail $sourcedata = getimagesize($sourcefile); /* now determine the dimensions of the new image - there's probably a more elegant way to do this */ if ( $sourcedata[0] > $sourcedata[1] ) { // if width greater than height $newwidth = $maxdim; $newheight = $sourcedata[1] / ($sourcedata[0]/$maxdim) ; } else { $newheight = $maxdim ; $newwidth = $sourcedata[0] / ($sourcedata[1]/$maxdim) ; } SWITCH ($sourcedata[2]) { // create resource from source image CASE 2: $imagesrc = @imagecreatefromjpeg($sourcefile); break; CASE 3: $imagesrc = @imagecreatefrompng($sourcefile); break; } /* create resource for a new, blank image */ $thumbnail = imagecreatetruecolor($newwidth, $newheight); /* now we'll copy one image to the other, resampling as we do */ imagecopyresampled($thumbnail,$imagesrc,0,0,0,0,$newwidth,$newheight,$sourc edata[0],$sourcedata[1]); /* the resource $thumbnail now contains the new image, so all we need do is output it */ imagejpeg($thumbnail); ?> Like I say, this all works fine on my local system, so I'm baffled. All ideas gratefully received.