Attach Files and Images Box Checked: This means that ALL images will be attached to the email so the viewer sees the entire message along with any images when message is opened.  The disadvantage is that tracking of who viewed the message will be inaccurate. This is because the viewer is not compelled to download the tracking image. Box Unchecked: This means that when a viewer opens a message the text of the message is visible; however,  images, logos, etc. will not be automatically displayed. The viewer's email client will automatically ask the viewer if they wish to download the images. When the viewer says yes, the images are visible. The advantage of this approach is that the viewer is compelled to download the tracking image causing the 'viewed' count in the job log to be more accurate.