How to replace a function using a child theme?

You should wrap your function in parent theme like this, not in child theme function.php

if (!function_exists('TaskerDev_get_first_post_image_fnc') ){
function TaskerDev_get_first_post_image_fnc($pid, $w = 100, $h = 100)
    {
//Parent theme code here
 }
}

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