This is mostly a PHP general question about concatenating a function’s returned value with a (or many) string(s).
The get_bloginfo function returns a string instead of echoing it directly.
Thus you start off with:
// First param value: stylesheet_directory
// Using "template_url" also works unless you are using a child theme
// In that case, "template_url" will not return your child theme's directory path
$image = get_bloginfo('stylesheet_directory');
If you want to manually add a string to that value, you concatenate the get_bloginfo() returned value (of type string) by adding a dot (.) followed by either single or double quotes, like the following:
$image = get_bloginfo('stylesheet_directory') .'/images/images/default.png';