Save .jpeg in cache dir?

In order to save some resources, I want to save watermarked images to the cache dir.
Currently, I use PHP’s file_exists to check whether the image already has been watermarked, but my question is, is there a way to save this file using CakePHP’s Cache class?
I want to save the file to /tmp/cache/img/filename.jpeg and load it from there as well once it has been saved.

Do you want to serve the pictures directly or through PHP?

Through PHP: just use the Cache class as with every other blob of data. Don’t bother with files, Cache will take care of that.

Directly: work with CakePHP’s File class and put them somewhere in the webroot folder.

I want to serve them through PHP preferably.
Will using the Cache class clean-up itself when the disk get’s quite full?