I have a node where two users have posted tens of thousands of images as attachments and I'd like to delete the nodes as they're not redundant. When I do so, will all those thousands of attachments be deleted as well?
Unused attachments are deleted automatically. They may belong to postings, postings belong to threads. When you delete a thread permanently (not a soft-delete), all attachments within in will be deleted as well.
Thread, that belong to a node, that you delete, will be deleted as well on node deletion.