The [IMG] tag is only useful if you want to link an image that is located on an external server (possibly your own). I don't recommend this because if your server goes down or your domain disappears then the LAVA forums will be left with a broken image. Also, large images can potentially make the web page become VERY wide and this is not desired. On top of that, it means people with slow connections will have to wait for large images to download instead of small thumbnails.

What I recommend is to use the built-in uploading and posting capability within the LAVA Forums. At the bottom of your post you will see:
After you browse to the image on your local hard drive, click on the "UPLOAD" button. We currently support JPG, PNG and GIF file formats:

You can do this as many times as you have images to upload. Once you are done with your sigle or multiple upload, you will see the following:
Now if that is all you need then you are done! You can click on the "submit Post" button and all the attachments will appear on the bottom of your post as nice scaled thumbnails.

Now if you want to get fancy... You can insert your images at specific locations within your post (just like I did above). All you have to do is place your cursor at the location where you want the attached image placed and then click on the "Insert attachment into text editor" button. This will insert a special code in your text body that defines the image: