I think you'll find that the captcha image will not be cached anyway.
There's various methods for a webpage to tell the browser not to cache an image and i'd suspect an image such as a captcha image will use some method to ensure it is not cached.
A possible solution is to inject some javascript into the webpage.
The javascript could get a reference to the captcha image, convert it to a base64 encoded string and then pass that string to a Sub in your b4a code.
There's some example javascript on this page: http://stackoverflow.com/questions/934012/get-image-data-in-javascript