Caching thumbnails

I'm writing an addon which will scrap various thumbnails from a popular website. I don't want to overload the site so I only want to scrape the files once or again if they go missing/are deleted.

I can download them to a file e.g. "mypluginname_uniqueid_small.jpg" and stuff it in some private directory but it seems like a bodge.

Is there any way to utilise XBMC's thumbnail cache easily with these filenames from Python? I can compute the unique id and ask to load the filename, then if I get a "file not found in cache" error, I can download the file. If the thumb file exists, I need to be able to pass it on to ListView to use. And, I need to be able to store new files in the cache.

I've looked over the API docs and can't see how it's done.

Thanks