If you are posting a translation, and wish to post a transcription of the original language version as well, it comes after the translation under an <lj-spoiler> tag.
Don't stretch the page!
Embedded videos should not exceed 430px in width. If the video is from youtube, this means choosing the 240p option or, they also give you the option of choosing your own size when getting the embed code
Scans should posted as thumbnails within the entry. The easiest way to do this is to upload directly to LJ, and when selecting images from album to post in the entry, make sure to check "Add a link to the original image" and Resize to "100" in the dropdown menu. But of course you're free to use any image service you like.
Members are free to tag their own posts, but use your discretion and check out what other tags are available first. Don't feel bad if any of the fields (era, media, member, source, translation, type, year) are unknown or inapplicable.