Вы используете класс phpThumb?
phpThumb.config.php
// * Error message configuration
$PHPTHUMB_CONFIG['error_image_width'] = 400; // default width for error images
$PHPTHUMB_CONFIG['error_image_height'] = 100; // default height for error images
$PHPTHUMB_CONFIG['error_message_image_default'] = ''; // Set this to the name of a generic error image (e.g. '/images/error.png') that you want displayed in place of any error message that may occur. This setting is overridden by the 'err' parameter, which does the same thing.
$PHPTHUMB_CONFIG['error_bgcolor'] = 'CCCCFF'; // background color of error message images
$PHPTHUMB_CONFIG['error_textcolor'] = 'FF0000'; // color of text in error messages
$PHPTHUMB_CONFIG['error_fontsize'] = 1; // size of text in error messages, from 1 (smallest) to 5 (largest)
$PHPTHUMB_CONFIG['error_die_on_error'] = true; // die with error message on any fatal error (recommended with standalone phpThumb.php)
$PHPTHUMB_CONFIG['error_silent_die_on_error'] = false; // simply die with no output of any kind on fatal errors (not recommended)
$PHPTHUMB_CONFIG['error_die_on_source_failure'] = true; // die with error message if source image cannot be processed by phpThumb() (usually because source image is corrupt in some way). If false the source image will be passed through unprocessed, if true (default) an error message will be displayed.