Click on 'Build Your Own' campaign to use our template editor.
Once you reach the page with the template editor, click on the small image (
) to the right of the HTML icon. This will give you the ability to upload an image from your computer. Here is what the "Image Manager" window looks like:

A pop up window will appear and your Image Library will display. Click on 'Upload' in the top right corner to add a new image to your library. An upload window will pop up and allow you to select an image from your computer.

To insert an image, just click on it.