There's no secret to posting pictures. You need a picture HOSTED somewhere on the web - i.e. on a server that is connected to the internet.
Then, to display the picture on ar15.com, you type in the http address of that picture on the server(like: www.whatever.com/picture.jpg) INSIDE the "image tags" - in other words you put {img} and {/img} around the address, and then it shows up as a picture.
Except instead of the { squiggly brackets, you use the [ square brackets around the img and /img terms.
Does that make any sense?
Team Members on ar15.com each get some storage space on the ar15.com servers where they can host pictures - but you can also host pictures on free sites such as photobucket.com, I believe.
For example - here is a picture I have hosted on the ar15.com servers:
{img}http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=37544{/img}
If I use the square brackets instead of the squiggly ones, it looks like this: