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Easy approach:
Click on this:

and type in you image addres.
To host your images online use
imageshack.
To link them use this button:

. To unlink use

.
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OR
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Use this syntax:
Code:
[IMG]location of immage here[/IMG]
eg:
Code:
[IMG]http://img80.imageshack.us/img80/5172/ubuntudarkbluezx7.th.png[/ IMG]
Here's how the above syntax would look:
you can also combine it with the [url] tag to acive clickable linked images
eg:
Code:
[url=http://img80.imageshack.us/my.php?image=ubuntudarkbluezx7.png][IMG]http://img80.imageshack.us/img80/5172/ubuntudarkbluezx7.th.png[/ IMG][/ URL]
Heres how it apears:
Note: i have intentionaly deformed the syntax in the code boxes so that the code and not the image is displayed.
I have inserted a space between "/" and the tag like: [/ IMG].
It should be [/IMG].
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Screenshot:
imageshack links screen:

Also read
this
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Hope this helps.