I've tried finding a 'howto' or 'welcome' page for iichan newbies, but have had no success. The place here looks very interesting, but I'm feeling a little lost. Has someone written an introduction or manual for image boards like this?

Thanks in advance!

There is the documentation on wakaba's and kareha's website (http://wakaba.c3.cx/docs/docs.html) but that one's a little counter-intuitive for newbies. 4-ch does have a FAQ (http://4-ch.net/faq.html) but that one mostly applies to the discussion boards.
4chan also has a FAQ but that one's very biased and sometimes misleading.

Writing an own FAQ for iichan/wakachan would probably be a good idea. dmpk2k is the only one of the admin team I'd trust with that, though.

Most of the following informations can be looked up on the official TiddlyWiki for Wakaba and Kareha:
http://wakaba.c3.cx/docs/docs.html

There is also a guide section for users new to these kinds of software:
http://wakaba.c3.cx/docs/docs.html#UsersGuide

If, after reading the guide and this text, there are still open questions and problems, please consider
taking the points you want to make and the questions you have to the official support forum:
http://wakaba.c3.cx/sup/index.html

USER GUIDE TO WAKABA AND KAREHA BASED BOARDS

• Name:

Your name goes here! You can also use tripcodes or secure tripcodes
(http://wakaba.c3.cx/docs/docs.html#TripCodes
http://wakaba.c3.cx/docs/docs.html#SecureTripCodes).

But you don't have to enter a name at all! You can just post anonymously.
There's good reason for posting as Anonymous as well as posting with a
name and even with a tripcode. Let's not pass judgement before gathering
experience how the boards work and what is best for the community!

• E-Mail / Link:

As of now, you can put three different things in here:

sage: If "sage" appears anywhere in this field, the reply will not bump the thread
which you are replying to the top. Again, this has different uses. Please note that
not all replies are useful for the community and also that most image boards have a
treshold of replies per threads after which a thread cannot be bumped anymore,
regardless of whether sage is used or not.

email adress: e.g. "snacks@5chan.org" will result in a "mailto:snacks@5chan.org" link.

hyperlink: e.g. "http://www.2ch.net" will result in a "http://www.2ch.net" hyperlink.

[Trivia]
There are also other possible uses which Kareha and Wakaba do not support (yet).
One of them is "fusianasan" (enabled on 2channel and other forums) which reveals
the IP of the person posting.

• Subject:

The subject of your post. Especially on discussion boards it is advised if you start
a new thread that a title is chosen that will help the community. Criteria for this
could be ease of searching for threads relevant to the users' interest as well as
pointing out continuity of certain discussion strings.

Good and bad examples:

"I was just thinking that..." <- Doesn't tell the user anything about the content of the thread.
"30 years old and NO friends" <- At least informative.
"[Pity] Your most awful relationship experience [Emo]" <- Categorizing by [theme] tags.
"[LOL] I just made a poo poo - Part 12 [DQN]" <- Indicates famous past threads with familiar subject.

Again, these are just considerations, no rules. However, the more you learn about
how the community works, the better you will be able to put your own considerations to use.
So, let's lurk and learn!

• Comment:

Anything you want to tell the world. Please abide by the specific rules of the board you
are posting in. Also take note that most Wakaba and Kareha boards enable you to markup
your comments quite a bit with Wakabamark: http://wakaba.c3.cx/docs/docs.html#WakabaMark

• File:

If you want to upload a file relevant to the community's interest, please do so!
Note that most boards specify the supported file types as well as specific restrictions like file size
under the "New Thread" form at the top of each page. Please also note that most image boards
by default require you to upload an image with the first post when starting a new thread and
that when you get a "No file selected" error you are trying to start a new thread instead of
replying to an existing one. If you want to start a new thread without uploading a file, just check
the [No File] checkbox next to the [Browse...] button.

• Verification:

Some boards protect themselves from flooders, scriptkiddies and spambots by enabling
verification codes. These are more or less random codes automatically generated by
the board's software. If enabled, you will have to enter what you see in the generated image
in the box next to it in order to be able to make a post.

• Password:

Required on image boards if you later want to delete a post. These will be stored in a cookie, if
cookies are enabled on your browser. Also, most of the time you don't have to choose a password
yourself because Wakaba comes up with one for you automatically if you have JavaScript enabled.

Maybe a link to /soc/ should also be included:
http://wakaba.c3.cx/soc/index.html

• What happens to old images? Are they kept somewhere?

Nope. Imageboard content is temporary, and when threads get old they will be deleted.

This is great. Thanks for the help!

This was put onto the wiki as well: http://4-ch.net/wiki/guide_to_posting