There are two ways to post an image...
If you want to integrate an image whereever in the text it fits, use the "img"-tags and point to the URL of the image. (If it is on your computer, use imageshack, photobucket or any other propper image-host out there in the web!)
Test image with the adress http://blog.vhs.at/manila/model/images/imgTestbild.gif

the way to do is: "img"Link"/img"
With this option, you can also place a link to an other page (which is quite annoying, but sometimes useful)

here, you have to use the url-tags.
"URL=Link""IMG"Link"/IMG"
(replace "" by [] or simply hit the buttons in the bar)
Now the second way to do it: Directly upload on Indigo!
Down the page, there is an "Add an Attachment"-section. (To see it, you need to click "post reply" instead of "quick reply")
The advantage is, that you can upload other things besides images (see Allowed Extensions and Sizes)
The disadvantage is, that it's ALWAYS on bottom of the page, then.
Where you can read "Filename", put in the path of your file or press the "Browse..." button
below, you can add a comment, if you wish.
after that, press "Add attachment" and it'll be uploaded.