First, take a look at Derek's post about resizing images.
When you open the new-post editing window, scroll down and you'll see a "Manage Attachments" button. Click on that, and you'll be able to upload images from your computer - assuming they conform to whatever size limitations the forum software imposes.
When you open the new-post editing window, scroll down and you'll see a "Manage Attachments" button. Click on that, and you'll be able to upload images from your computer - assuming they conform to whatever size limitations the forum software imposes.