Like the 3D wave captcha, the GD captcha seems to have the same issues with mt_rand, from my error log:

Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-5) is smaller than min(0)
Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-7) is smaller than min(0)
Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-8) is smaller than min(0)
Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-11) is smaller than min(0)
Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-17) is smaller than min(0)
Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-27) is smaller than min(0)
Anonymous 222.166.181.207 Sun Feb 13, 2011 10:56 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-5) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-7) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-9) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-10) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-13) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-18) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-27) is smaller than min(0)
Anonymous 222.166.181.11 Sun Feb 13, 2011 10:55 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-41) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-4) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-5) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-6) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-8) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-10) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-16) is smaller than min(0)
Anonymous 222.166.181.41 Sun Feb 13, 2011 10:54 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-24) is smaller than min(0)
Anonymous 222.166.181.23 Sun Feb 13, 2011 10:53 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-1) is smaller than min(0)
Anonymous 222.166.181.23 Sun Feb 13, 2011 10:53 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-1) is smaller than min(0)
Anonymous 91.212.226.207 Sun Feb 13, 2011 7:45 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-4) is smaller than min(0)
Anonymous 91.212.226.207 Sun Feb 13, 2011 7:45 am Error while creating image
» Error in /includes/captcha/captcha_gd.php on line 80: mt_rand(): max(-5) is smaller than min(0)

It seems that it doesnt happen every captcha call, as when I try to see the image, the image is still displayed.