Hi! It isn't possible to add an image without a link; your image must be hosted on another site.
Some good sites for hosting include filegarden, postimages, and freeimage.host! And to embed an image into a post, all you need to do is use this code:
[img src="YOURLINKHERE.png" style="max-height=100px">
(just replace the [ bracket at the start with a < lesser than symbol, I couldn't get the code to display without it aha)
Just put your image link in the parentheses, and you can adjust the size of the image in the thread by changing the number of pixels! (or you can remove the style part entirely and it'll just default to the images normal size)