Use the following code in your theme’s functions file to change the default image.
function mytheme_megacal_default_image( $path ) {
return 'https://yourwebsite.com/wp-content/uploads/defaultEventReplacement.png';
}
add_filter( 'megacal_default_event_image_path', 'mytheme_megacal_default_image' );
Just remember to change the filename and path to the image you choose.