Captcha Image Error

Hi,

my captcha image is not showing. It is not htaccess or php sessions error but when I checked the server logs it gives me this error:

libpng warning: Application built with libpng-1.2.51 but running with 1.5.15
PHP Warning:  imagepng(): gd-png error: cannot allocate libpng main struct in /home/.../public_html/wp-content/plugins/si-captcha-for-wordpress/captcha/securimage.php on line 903

Any idea how to fix this?

Thanks!

https://ww.wp.xz.cn/plugins/si-contact-form/