I've mentioned a lot of this stuff in previous threads, and I don't know that a lot of people are going to read this thread anyway, but what the heck; here are some suggestions that I'd like to give users who are interested in using image tags to make images on KYM easier to find.
First, use tags. This should be obvious if you're bothering to read this thread, but I'll say it anyway. An image that is not tagged, has no title, and isn't attached to an entry will probably never be found by anyone. While not every image goes with a specific entry, every image can potentially be described, so use tags.
Be thorough. I don't know how many people realize it, but searching for the tag ermahgerd won't give you images from the entry Ermahgerd that don't carry that tag, so I always tag with the name of the entry. If you know the names of the people/characters in the picture, add them as tags, and use their full name if applicable (see below). If there is a notable object, theme, or action in the picture, make a tag for that. If it's fan art, and you know the artist's name, it just seems polite to add it.
Be efficient. Fun fact: If you tag an image with "My Little Pony: Friendship is Magic" then people will get the image if they search for "My Little Pony", "Friendship is Magic", "Pony", "Magic", and even "friends" or "ITT". This is why tagging a full name is generally better, because if you add the tag "Justin Bieber", the tag "Bieber" becomes redundant, so don't bother. And if you're trying to decide whether an image of Misha should have the tag "Wahaha" or "Wahahaha", remember that the latter will pick up searches for the former. Although of course you should consider that an MLP image could probably also use the tag "MLP:FIM". Closely related:
Don't over-tag. I see a lot of picture taggers doing things like putting the tag "Know Your Meme" on photos. Really, this should only be a tag on an image that is actually about Know Your Meme, like this or this. Also, as I was discussing with RandomMan a while back, I put the tag "meme" on images that are talking about memes, not on images that are memes, because the latter category is 99% of the database. That's general, but in a more specific case, if you're tagging a picture of Zoidberg, then "Zoidberg" and "Futurama" are reasonable tags, but "Fry", "Leela", "Bender", and "Farnsworth" really should only be there if those characters actually appear, or at least are mentioned in the caption. Also, don't tag with your user name (unless it's your personal fan art, I guess, or a picture of you). Searches for "Brucker" turn up images that I uploaded anyway (under that username); it will do the same for you.
Is there anything that I missed?
ETA: See James' post below for advanced image searching tips.