Hi, Punya (and Ayu
The nearest thing to a 'user tutorial' at the moment is the FAQ's page http://dharmawheel.net/faq.php
accessed via the link near the sign-in link (bottom right of page header).
As for resizing images, I'm pretty sure you can't do it within the forum's upload process. I always do it on my own computer before uploading them, using Preview (Apple's equivalent of Picture Viewer but better) or Picasa (free software for all operating systems - see http://en.wikipedia.org/wiki/Picasa
). There are lots of other image editing programmes out there, many of them free.
There are also online apps which will do it for you. I've never used Photobucket (http://photobucket.com
) but I have often seen it recommended. I do use Flickr, primarily to share photos with others but it does automatically resize photos that are uploaded to it and you can share the smaller versions as easily as the original size.
Video? I have never tried except to share stuff which is already online, mostly YouTube. It is then just a matter of posting a link, between either [url][/url] or [video][/video] tags. I think the size limits on uploaded attachments would stop you uploading video from your own computer but it might just work if you had a very short, low-resolution clip.
Hope this helps,
P.S. I think Flickr allows the uploading and sharing of video as well as single images. If so, that could be the easiest solution to sharing your own video.