Well first the file has to be somewhere that it can be referenced. Either on your site or some other site that cann be accessed via the internet.
One thing you could do is add the file manager feature and upload the image per post. But then also you would need to make changes in the control panel.
You can try a test upload post in the test forum here.
https://www.ubbcentral.com/forums/ubbthreads.php/topics/220537/Test_image_post.html#Post220537
