Since Attachments are turned off you have to do following:
1. Sign in (if you still don't have account) one of the image hosting sites (Imageshack, Flickr, Glowfoto, etc.).
2. Upload pictures you want from your computer.
3. Copy link of the picture in your SciForums post by:
3.a. Using 'Image' icon (yellow one with mountain and Sun in it)
3.b. Pasting link in message window and then wrapping it in IMG tags (using [ bracket)
4. 'Submit reply' (First 'Preview post')