Caching generated images on the fly

I’ve written a plugin that generates an image on the fly, using some values stored as WP options. It works fine, but I was wondering how I should improve performance.

The image is generated with GD and sent directly to the browser. It does not exist as a real image (in a file) or as an attachment.

One thought is to save the image data in a transient. But, it appears as though transients can not handle binary data.

Can anyone recommend an approach? thanks.