Screenshots for manga are supposed to be single pages of the manga, so just extract a few pages that are representative of the quality of the rest of the torrent. Do not obtain images from elsewhere, because they may not be the same quality.
About the archiving, I personally think it's fine, but the norm seems to be having it packaged up. I'd say, as a rule of thumb, to keep the data in the same form as was released by the original translators. If they archived it, do not extract them. If they had a torrent without an archive, don't archive it. This way, people who want to reseed a manga torrent can obtain the files from the original uploader, whereas if you create an archive, it will not be the exact same file as was originally released unless you happen to create it with the exact same settings using the exact same program on the exact same operating system.