Widget: EZG Recent folders

You can use the eazyest_gallery_folder_icon filter.

The plugin uses that filter for the folder icon:
$icon = apply_filters( ‘eazyest_gallery_folder_icon’, eazyest_gallery()->plugin_url . ‘frontend/images/folder-icon.png’ );

WordPress page about how to add filters:
https://codex.www.ads-software.com/Function_Reference/add_filter

The code for your functions.php:

// Custom folder icon thumbnail
function custom_folder_icon() {
	return 'custom/path/to/image.jpg';
}
add_filter('eazyest_gallery_folder_icon','custom_folder_icon');