Plugin a specific cache functionality?

I have written a plugin that puts a (google) favicon in front of each link in my blog. Really simple. Just uses a simple preg_replace_callback on hrefs:

$changed_html_reference = preg_replace_callback(self::HTML_REF_REGEX, 
   array($this,'AddExtraHtmlToOneHref'), $strHtmlBodyText);

with a

add_filter('the_content', array($this,'ReplaceAll'), 9);

for the replacement and a default call to the google site for the icon (snip out of class) :

const GOOGLE_ICON_URL = 'http://www.google.com/s2/favicons?domain=';
function HttpDownloadFeed()
{
 $parsed_url = parse_url($this->url);  
 $data_r = wp_remote_get(self::GOOGLE_ICON_URL . $parsed_url['host']);
 $data = $data_r['body'];
 return $data;
}

I now have taken the approach of making my own cache class which stores the icons in directories like e.g. /cache/com/facebook/www/f.png.

But I am now thinking about the location of that class. For easiness i placed the cache in the wp-content directory. The following questions I have:

• could i plugin an existing cache / cache plugin to do the caching for me?
• what is the best location for my own cache thing? should i put it under /wp-content/cache or /plugins/myplugin/mycache or even /themes/mytheme/cache ?

or is there even a better approach to this?

(I am using the com/facebook/www approach because i also store session date of stumbleupon, delicious etc…in there and i only want to call these pages/feeds once incl. the request for a google favicon) (and since i need to display the icon and not all browser support inline displaying of icons embedded in html i need to write them to a directory available to the client).