nelson on "Different sizes download button"

Hi,
I have a requirement from a client, he needs two buttons in the page for two different sizes of the current image, those two buttons would allow the user to download such images, different from the original image size
I'm using a custom template, in there, I used getCustomImageURL() for both links; the returned value from getCustomImageURL is sent to a custom php script which sends the image to the browser.
It works great, and the user can download the two different sizes; BUT ONLY if those two sizes had been viewed before, that is, if they are in the cache already. If the image was viewed at the custom size, I get the URL to the cache file, but if not, I get the URL to the image processor
How can I make getCustomImageURL return the URL only after the image processor has finished with it (in cache) ?

Using ZPhoto 1.4

Thanks for any pointer to a solution

Nelson.