Do you have an image without the girls face and reply bubble? I've tried reverse image searching with little to no success: Bing believes the dog looks like a Canadian Eskimo Dog or Siberian Husky, here's tools for reverse image search:
◻Google and Google search by file name
◻Tineye
◻Bing
◻Yandex
◻SauceNAO
Download the photos the poster uploaded quickly! There may be metadata within that can help. Don't change the name of the files because they may contain helpful information. Download all the photos not just screenshot (though those help), save the pages (https://wikihow.com/Save-a-Webpage) you don't know if the poster will delete or hide evidence. More saved content is better than less and you can't go back in time to obtain lost posts.
The metadata in photos is what helped people track down, identify, and arrest Luka Magnotta. I was wrong: according to "Don't F**k with Cats: Hunting an Internet Killer" someone informed the Internet sleuths that the person in the video was Magnotta which they had failed to figure out on their own. Have you tried reporting to the police in your area that your missing friends account is suddenly active again? Have you told her family?
8kun is not a reliable source for images it will 404 most uploaded images. Use https://imgur.com (now .io) or https://catbox.moe