You can attach them. To do so, click on the + Additional options (lower left) and select "attach", then "choose file"...and browse to the image you wish to attach from your hard drive. That allows you to attach 4 images. They will be small, and require the viewer to click on them to see them fully.
You can "link" to them off the Web.
To do so...find the image on the Web. Right click on the image and "copy image URL".
To do so, click the "image" icon in the tool bar above. It's the second one from the left, second line down (next to the circle thingy) and looks like a light blue photo. When you click on that the "tabs/brackets".....and right in the middle, which is where it automatically puts your cursor, just paste (Control V).
I'm using both methods here.
Hope this helps.