[Mooff]

edited the same time you made your post.

edit: another option would be disableing the image resize process. Then you should make sure that your attachments are allready uploaded in the size the slider does need.

That would be (untested):

replace

               //rebuild image to the height and width we want in the slider 
                //called sprite since i got the code from our sprite addon :P
                $sprite = imagecreatetruecolor($maxwidth, $maxheight);
                if(is_resource($sprite) AND $article['previewimage'] )
                {                   				
                    $imageinfo = getimagesize($article['previewimage']);
                    if(is_array($imageinfo))
                    {   
                        $image = null;
                        switch($imageinfo[2])
                        {
                            case IMAGETYPE_PNG:
                            $image = imagecreatefrompng($article['previewimage']);
                            break;
                            case IMAGETYPE_GIF:
                            $image = imagecreatefromgif($article['previewimage']);
                            break;
                            case IMAGETYPE_JPEG:
                            $image = imagecreatefromjpeg($article['previewimage']);
                            break;
                            default:
                            echo '<span style="color:red">Error occurred:</span> Unknown image format. ' . $article['previewimage']. '<br />';
                            break;                        
                        }
                        
                        if(!is_resource($image))
                        {
                            //resiziing did not work - we are using the fallback image.
                            $article['previewimage'] = $fallback_img;
                            echo '<span style="color:red">Error occurred:</span> imagecreation failed. ' . $article['previewimage']. '<br />';
                        }
                        else
                        {
                            $img_width = $imageinfo[0];
                            $img_height = $imageinfo[1];
                            
                            imagecopyresampled($sprite, $image, 0, 0, 0, 0, $maxwidth, $maxheight, $img_width, $img_height );
                            imagedestroy($image);

                            $img_filepath = $slider_img_filepath . 'slide_' . $i . '.jpg';
                            $success = imagejpeg($sprite, DIR  . ($img_filepath[0] != DIRECTORY_SEPARATOR ?  DIRECTORY_SEPARATOR : '') . $img_filepath);
                            imagedestroy($sprite);
							if($success)
							{
								$article['previewimage'] = $img_filepath;
							}
							else
							{
								$img_filepath = DIR  . ($img_filepath[0] != DIRECTORY_SEPARATOR ?  DIRECTORY_SEPARATOR : '') . $img_filepath;
								echo '<span style="color:red">Error occurred:</span> imagejpeg failed. ' . $article['previewimage']. '<br />
								directory: '. $img_filepath ;
								
								$article['previewimage'] = $fallback_img;
							}
                        }
                        
                    }
                    else
                    {
                        //resiziing did not work - we are using the fallback image.
                        $article['previewimage'] = $fallback_img;
                        echo '<span style="color:red">Error occurred:</span> picture is not readable.' . $article['previewimage']. '<br /> ';
                    }                  
                }
                else
                {
                    //resiziing did not work - we are using the fallback image.
                    $article['previewimage'] = $fallback_img;
                    echo '<span style="color:red">Warning:</span> article has no previewimage or sprite is no ressource. ' . $article['previewimage']. '<br />';
                }

with

                if($article['previewimage'] )
                {      
                    //do nothing
                }
                else
                {
                    $article['previewimage'] = $fallback_img;
                    echo '<span style="color:red">Warning:</span> article has no previewimage or sprite is no ressource. ' . $article['previewimage']. '<br />';
                }

Pictures should show up then. But it might look odd if the pictures do not have the right width and height.