There is a bug with the post editor. Images pasted from other websites from your clipboard will automatically use the [img] tag instead of uploading a copy as an attachment. Please manually save the image, upload it to the site, and then insert it as a thumbnail instead if you experience this.
The [img] should essentially never be used outside of chat. It does not save disk space on the server because we use an image proxy to protect your IP address and to ensure people do not rely on bad third party services like Imgur for image hosting. I hope to have a fix from XF soon.