Trying to add cat_ID to array for showing an image for specific Category

I have not used that plug-in but I would guess that your function get_the_image() doesn’t accept that parameter as part of the array.

I would first check for that custom function to be active. If you turn that plug-in off you will get an error on your page:

<?php if ( function_exists( 'get_the_image' ) ) { get_the_image(); } ?>

Then just insert your parameters as above.

You can try this (untested) is this in the loop? if not this might not work for you.

<?php 
$category = get_the_category(); 
if ( function_exists( 'get_the_image' ) ) get_the_image( array( 'meta_key' => 'feature_img', 'size' => 'medium', 'width' => '300', 'height' => '170', 'image_class' => 'feature', 'default_image' => 'http://www...../uploads/img_cat_' . $category[0]->cat_name . '.jpg' ) ); 
?>