Not sure how you can tell Xinha to the use the image if the URL of the image contains the item_id of the object the file belongs to (e.g. the question), which will only be created once the question exists which is after the download.
If the item_id is the item_id of the file, then you can use just plain /o access, but this would prevent you from reusing images that have been uploaded to an item (e.g. if 5 images have 12345 as the parent item_id, calling /images/12345/ could result in an UI that shows you all the images stored with 12345).
I definitely like the clipboard idea and it might be easier to come by:
You could upload images, mark them as private (then the parent_id is your user_id), package (then the parent_id is the package_id of the package calling XinHA) or site_wide making it available site wide. The insert image button in Xinha would then look at the clipboard and display all images in the clipboard from which you can choose to upload and insert.
