Just a word of caution: even if they eventually support that feature, keep in mind that allowing arbitrary image uploads without any sort of attribution can be a huge liability, and it's likely to be abused. I'd personally be in favor of keeping hardcoded checks within XenForo to prevent guest uploads; if someone truly understands the implications of allowing guest uploads, they'll be able to remove those checks on their own.
Thanks for your constructive thought behind this, P, as it made me rethink my position on it. I originally based it on the premise that permissions had a maximum limit of attachments per post. However, it appears that you can only turn attachments on or off. There ought to be an add on out there already that limits this; and if not, it doesn't seem like such a hard process to implement.
On the other hand, there is a maximum attachment file size.
If you are able to lower the number of attachments for guests—through an existing add on, custom development, or future implementation of a permission—this would take care of the abuse issue. Furthermore, if you combine the permission with a file compression after upload, it would be even less of a problem as the file (specifically speaking an image here) would be compressed and resized on the server-side to not keep a huge footprint; but as a bonus, you would gain a possible contributing member.
Nonetheless, we are talking about post before registering here. If a guest wants to attach an image, the "Move" should be removed as it implies that an image can be uploaded or the functionality should be added and permissions set to not allow uploads to guests if that is your stance.
You may drive someone crazy if they think they can upload when in fact they can't because of that "Move" being there.
Therefore, it has to come down to removing the tooltip or appearance that a file can be uploaded (move into the box) or to allow guests to post attachments.