Themes: in admin, themes, this is the message I get - please help

Failed: setSourceFilename(/usr/www/users/swampc/plogger/plog-content/themes/air/preview.png) set $this->sourceFilename to "/usr/www/users/swampc/plogger/plog-content/themes/air/preview.png" in file "phpthumb.class.php" on line 214 $AvailableImageOutputFormats = array(text;wbmp;gif;png;jpeg) in file "phpthumb.class.php" on line 630 $this->thumbnailFormat set to $this->config_output_format "jpeg" in file "phpthumb.class.php" on line 640 $this->thumbnailQuality set to "75" in file "phpthumb.class.php" on line 657 $this->sourceFilename set to "/usr/www/users/swampc/plogger/plog-content/themes/air/preview.png" in file "phpthumb.class.php" on line 581 $this->config_cache_directory () is not a directory in file "phpthumb.class.php" on line 698 SetCacheFilename() failed because $this->config_cache_directory is empty in file "phpthumb.class.php" on line 2009 EXIF thumbnail extraction: (size=0; type=; x) in file "phpthumb.class.php" on line 1949 starting SourceImageToGD() in file "phpthumb.class.php" on line 2095 Not using EXIF thumbnail data because $this->config_use_exif_thumbnail_for_speed is FALSE in file "phpthumb.class.php" on line 2098 Calling ImageCreateFromPNG(/usr/www/users/swampc/plogger/plog-content/themes/air/preview.png) in file "phpthumb.class.php" on line 2160 ImageCreateFromPNG() was attempted but FAILED in file "phpthumb.class.php" on line 2201 Populating $this->rawImageData and attempting ImageCreateFromStringReplacement() in file "phpthumb.class.php" on line 2204 phpThumb() v1.6.2-200511240855 Failed to generate phpThumb_tempnam() in /usr/www/users/swampc/plogger/plog-includes/lib/phpthumb/phpthumb.class.php on line 2681 You may need to set $PHPTHUMB_CONFIG[temp_directory] in phpThumb.config.php $this->gdimg_source is still empty in file "phpthumb.class.php" on line 2227 using ImageMagick path from `which convert` (/usr/bin/convert) in file "phpthumb.class.php" on line 818 ImageMagickThumbnailToGD() aborting, phpThumb_tempnam() failed in file "phpthumb.class.php" on line 913 ImageMagickThumbnailToGD() aborting because ImageMagickCommandlineBase() failed in file "phpthumb.class.php" on line 916 ImageMagickThumbnailToGD() failed in file "phpthumb.class.php" on line 2236 phpThumb() v1.6.2-200511240855 All attempts to create GD image source failed (source image probably corrupt), cannot generate thumbnail phpThumb() v1.6.2-200511240855 Unknown image type identified by "?PNG" ( 0x89 0x50 0x4e 0x47) in SourceImageToGD()