First, you have to Upload the Image to an Image Hosting website.
There are lots of Image Hosting websites online (Such as Tinypic.com).
A simple search will find them.
Once your Image is Uploaded on an Image Hosting website.
Copy the Direct Link for the Image.
To add this Image to an Email.
On the Create Campaign Page for the Email Campaign.
In the Body of Email section.
Click on the button near the top right corner that looks like a picture of a mountain.
A window will open.
Paste the Direct Link to the Image in the bar in the middle of the window.
Click the INSERT IMAGE button near the bottom right corner of the window.
The Image will now appear in the Body of the Email.