You need to return content from filter function.
function find_my_image( $content ) {
if( is_single() ) {
if ( preg_match('#(<img.*?>)#', $content, $result ) ){
$content .= '<p>Image has been found</p>';
}
else{
$content .= '<p>Sorry, no image here!</p>';
}
}
return $content;
}
add_filter( 'the_content', 'find_my_image' );