Hi Mike,
hmm - I've tested here using the BasicGraph example and it seems to be functioning ok.
One "hole" I can see though is if the web browser doesn't fetch the image. In that case it gets generated, but never served. If it's not served then of course it's not deleted.
There are 2 cases I can think of that might do this.
a) the user has "images" for their browser turned off or
b) there is a program, other than a browser, pinging your site. Gogglebot for example will probably visit your site from time to time, and I don't know if they'll bother to fetch the images. Also if someone points an RSS reader at your main page that also has the effect of fetching the text, but not necessarily the images.
So a couple questions for you to do -
a) do you get this effect in the example
b) do you get this effect when _you_ access your site via a browser, or are you just seeing images left behind by other users?
Cheers
Bruce