If you attach a photo using the "manage attachments" button below your post, it'll post with IMG tags. If you want to post a photo hosted elsewhere you can click the little "photo" icon
and just type in the URL where your image is hosted. Either way, it uses IMG tags by default. Normally, you want to change that to TIMG, because most people post ridiclously large photos that aren't suitable for many screens (13" - 15") on notebooks and such. To change that, you have to do it manually as far as I know ... just add a T to the IMG tag in the front and back. ie: [img= or [timg