There are two ways.
1. Click the 'insert image' icon on the top of the posting box (it's fourth from the right), and add the filename from your computer or the URL for an online image. Only five images per post, I think - easier for you, but less convenient for the reader and uses precious bandwidth, so I'd encourage you to:
2. Upload the images to a hosting site such as imagebam or pixhost, then copy and paste the BB-codes into your post to display the thumbnails - and the reader just clicks on the thumbnail to open the pic in a separate window. Follow your preferred site's instructions. Most of us are using imagebam now that the threat of closure has been lifted - if you're using Pixhost, please remember to tick the 'create gallery' box so we can scroll from one pic to the next rather than have to click on every thumbnail separately!
And happy posting - looking forward to seeing your new stuff.