To post pictures within your thread, do the following:

1) Put in a thread title and description
2) Before you hit submit, scroll down to "additional options"
3) Click the "manage attachments" button in the "Attach Files" section
Note1: Valid file extensions are: bmp doc gif jpe jpeg jpg pdf png psd txt zip
Note2: If you click the 'manage attachments' button and nothing happens, your pop-up blocker is preventing the attachments window from opening. If you fix that, you'll be able to attach pictures.
4) Click the "Choose file" button in the "Upload File from your Computer" section at the top of the pop up window.
5) Browse to the file on your computer, click it once to highlight it, and click the "open" button.
6) Click the "Upload" button on the right side of the "Upload File from your Computer" box
7) Repeat for each file you want to attach to your post. There's a maximum of about 10 you can attach to each post. If you have more, you'll need to reply to your own thread and post them in the reply.
8) If your pictures are too large, you may have to resize them. I use Microsoft Paint to open them, then do a control-w to "Stretch and Skew" them. If you put a number less than 100 in the horizontal and vertical stretch boxes, it will shrink the image. 50 and 50 will reduce the image by 50% of it's original size. Once you have done this, file/save the new file and you now have a smaller image.
8) Finally, click the "Submit Reply" button and poof, you've created a thread with attached pictures.
9) If you have troubles, contact a forum mod for assistance.

Additional information on how to use other picture hosting services can be found here: http://www.ar-15.co/forums/showthread.php?t=10978
Be sure to use the [img} tags [/img} (but with square brackets on each side) to make your image hosted elsewhere display correctly. For instance:
[img}http://www.ar-15.co/forums/images/misc/title_image.jpg[/img} replace the } brackets on the right side of the img tags with ] and this becomes

You'd do the same thing with the URL of your hosted image.

Again, if you have troubles, contact a forum mod for assistance.