I've noticed some of our users don't know how to attach images in the forum thread correctly, here is a quick tutorial about how to do that:

1. Create a new thread and click the image icon:
 

2. Choose file to upload:


3. Pick up a position where you want the images to be located, and click the image: 
 

4. The selected image will go to the thread body, un-selected image will be shown in the bottom as an attachment, if you don't need that un-selected image, go to the image uploading panel again, and click "delete" to delete the image.