Alright, here goes. This worked for me on diverse boards.
Getting your pictures on the internet. Look for some solid free hosting for some simple pictures.I generally user the www.xs.to site for this, although there are many alternatives. Once you upload an image there are usually 4 to 5 different URL's provided to your picture. Mostly they are URL's when adding your image to a board (like this), but one is a plain link. Copy and past that link somewhere (like notepad). It should be something like "http://xs.to/pictures/23988655784_picture.jpg" .
Create a post, like this one.
[ img] xxxxx [/img]
Substitute the xxxxx with the url copy-pasted to notepad, and make sure the space between the "[" and 'img]' in that first block is removed. I used the space to show you what needs to be typed. It should be something like:
[ img]http://xs.to/pictures/23988655784_picture.jpg[/img] (off course without the space, mentioned above).
Hit the preview button to check if the image was linked correctly. If not, something like this might appear :
This generally means that the link you provided did not work.
NOTE: This is the very basic version of adding images. For really big images I would recommend creating thumbnails. First try this, then see if you can figure out how to create thumbnails (they are much more elegant btw).\