What do you mean? Like what is at the top of this post or like this:
Well, for the one at the top of this post, there is an option under "posting style" (it varies location from guilds and regular forums) that says "Display Inventory". From there you can select items from your inventory to show. But it doesn't show all your items if you have a large inventory.
For the image to be actually in the post is fairly easy. Go into your inventory and right click the item you want to use. When the popup menu comes up you have two options to get the image url. One is to just copy the URL directly from the menu. The other one is if you can't do it from the menu is to open the image in a new tab and copy the url from the address bar. Once you get the url, go to your post and use the img code with the url.