Heres how to do it:
go to:
http://imgur.com/Step 1: Find the 'upload images' tab on the side (circled in red pen below) and select the 'computer' or 'web' upload button
Step 2: You may select as many images as you want. When you have found all your images, click on 'start upload'
Step 3: After your pictures finish uploading, you will be brought to this page. I highlited the link you need (BBCode). Copy it.
Step 4: in your 'post reply' or 'start new topic' message box, just paste the BBCode.
Should come out something like this:
All done!