diff --git a/bbs/gcaptcha/gcaptcha.lib.php b/bbs/gcaptcha/gcaptcha.lib.php
index b851964b4..2537f8ee7 100644
--- a/bbs/gcaptcha/gcaptcha.lib.php
+++ b/bbs/gcaptcha/gcaptcha.lib.php
@@ -122,7 +122,7 @@ class gcaptcha
 
         $this->captcha_filename = $this->get_captcha_filename();
 
-        imagepng($im, G4_DATA_PATH.'/cache/gcaptcha-'.$this->captcha_filename.'.png', 0, NULL);
+        imagejpeg($im, G4_DATA_PATH.'/cache/gcaptcha-'.$this->captcha_filename.'.jpg');
         imagedestroy($im);
 
         $this->make_wav($this->captcha_filename.'.wav');