Windows: right click on the picture, click "Properties", highlight and click on the information given under "image address"
Mac: right click on the picture, click on "Copy image address"
Back on BFnet: put your little blinky cursor thing where you want the image in your post, click the icon that looks like a yellow postcard, a small box will open up, paste the image address where indicated, click on OK.
EDIT: See how big this pic is? That`s what I don`t like about it- any way to resize images imported this way? The advantage is that you can post pictures, write, then more pics, then more text.