Sorry for the browser language, but the entire tutorial is in English.
1) Click on "browse"

2) Browse to the file, either
double click on it or
click on the open button.

3) Click on "host it!". Note that images that are too large(>1.5MB as of 2007) will not be uploaded.
You will then be directed to a place with links to the image.

You can click on any of the fields to select it. press
ctrl+c or
right click->copy to copy the field. you can then press
ctrl+v or
right click->paste at your post to insert the field.
If you want to post an image, copy the field next to "Thumbnail for forums (1)", and paste it in your post.
for example,
[URL=http://img223.imageshack.us/my.php?image=11329223011319zj.jpg][IMG]http://img223.imageshack.us/img223/1482/11329223011319zj.th.jpg[/IMG][/URL]
It will result in this:

If you want the entire image, paste the field next to "Hotlink for forums (1)"
for example,
[url=http://imageshack.us][img]http://img223.imageshack.us/img223/1482/11329223011319zj.jpg[/img][/url]
And will result in:

If you want a thumbnail that leads the image itself and not the site, replace the content of the
part with the field next to "Direct link to image".
for example:
[URL=http://img223.imageshack.us/my.php?image=11329223011319zj.jpg][IMG]http://img223.imageshack.us/img223/1482/11329223011319zj.th.jpg[/IMG][/URL]

to:
[URL=http://img223.imageshack.us/img223/1482/11329223011319zj.jpg][IMG]http://img223.imageshack.us/img223/1482/11329223011319zj.th.jpg[/IMG][/URL]

(click on the images to see the difference)
Extra:
- In the upload page, you can check\uncheck

.
it removed\add the the black info bar in the bottom of the thumbnail. The bar shows you the resolution of the image, and its size.

- You can resize the image, simply check "resize image" and choose the wanted resolution.
Though, this isn't very helpfull.
I wanted to give examples, but it seems the service doesn't work right now.
- Some images may not have thumbnail. this is because they are too small (under 200x200 px)
- You should not use sharing programs while uploading images, it may cause the uploading last too long, and the session will time out.
You can always try again, and it won't allways disturb the uploading. It also depends on how much upload bandwidth you have.
- You can tell ImageShack to upload images from other sites, so you won't have to download the image and then upload it back.
select url, and paste in the field the URL of the image.

click "host it!", and the rest is the same.
- You should read the FAQ.