Just put the image file on the forum like normal and highlight that whole text and use the link button to link it to the original image. I suggest imgur.com for this because you can embed small images.
[URL~"ORIGINAL IMAGE LINK"]
I struggle with this, too. The only way I know how to attach a larger image, but link thumbnails inline with my post, is to host them offsite. I use photobucket. I then copy link from PB back to LPF posts. The only downside is I can never move or organize the photos, because the links get broken in the posts.
Depends what site youre using to host the image file. Imgur gives you a link to use depending on how big you want the image to be, all using the same image you uploaded. You should probably use photobucket as pschlosser suggests it sounds easier.