Sub image ID is available in the URL for an image. For example, the experiment here shows imageID=265851306 in a screenshot below (boxed in red), which can then be used in an image download URL command like so:
http://api.brain-map.org/api/v2/image_download/265851306
There is also a post from another issue here, that describes image downloading for the Glioblastoma dataset more generally.