How to revert a Wiki edit?

The page https://wiki.librarything.com/index.php/HelpThing:New_pic was vandalised with spam on 14 July 2018‎, and apparently nobody has noticed until now.

That's sad enough, but even worse I can't figure out how to get back to the previous version.

Maybe there just isn't a previous version?

If you go to that wiki page and click on History in the left-hand column under This Page, it will show you the Revision History of the page. In this case, it appears this was a page that was created by the spammer themselves, so there's no previous version to revert to. I know anyone can edit a page, but I couldn't see any way for an ordinary LT user to delete a wiki page, though I didn't have time to explore very deeply.

We wouldn't want to delete that page in any case, it's the one linked to from the Help menu item on a picture page (e.g. https://www.librarything.com/pic/6767619).

Do you mean the picture guidelines:
https://wiki.librarything.com/index.php/Contributing_photos_to_LibraryThing

That goes to this link.

No, I mean the link which is at the top right of every page. From a picture page like the one I linked to, it takes you to https://wiki.librarything.com/index.php/HelpThing:New_pic

https://www.librarything.com/profile/Abdurahman_Saied9642
Well, here's the author in question.

(Though I wouldn't have changed the page until a LT admin had seen it.)

He was the author of the spam. Admins can still see what he wrote in the history. I have flagged this up to lorannen to see if they have any earlier versions.