You can attach images via URL if they are already hosted somewhere. Wiki pages can't use vBulletin attachments because vBulletin ties the attachments to a specific user account. This makes it impossible for multiple users to edit the same page properly once an attachment is added, and difficult to reuse attachments that someone else posted. Because of this, VaultWiki 4 has been given its own attachment system that, like pages, doesn't force that one belongs to any one user.
There were some bugs in Alpha 7 that made it impossible to create any page that counted as an attachment. This is fixed in build 2.
Also, if you are unable to upload anything, go to Admin Panel > Content > Attachments, and make sure the file extension you're trying to upload exists and is properly defined.
Please let me know what browser you are using also. Since the uploader uses Javascript it might be behaving differently in different browsers. I haven't had an experience where the displayed path is wrong...