Through the use of the CSS Override feature, you can change the background of your MegaEdit editor. This article will explain how to achieve this and the steps to take.
Upload your Image
1. Navigate to Content Management > CSS Overide.
2. Select

3. Press

4. Save the end of the URL for later use. If your image URL is:
/123456789/Handler/CSSOverride/GetImage/12/background.jpg
Then you will want to save:
/background.jpg
Change the Canvas Configuartion
1. Navigate to Configuration > Settings > Product and Editor Settings > MegaEdit product settings.
2. Press the Default Product Configuration tab.
3. Under Canvas, change these two settings exactly as they appear in the image below:

5. Press Save.
Upload CSS Code
1. Copy the below code, be sure to change the image reference according to your site (Bolded), use the reference from above.
.DynamicProductWrapper .canvas-container {
background-image: url('{images}/background.jpg');
background-size: cover;
background-position: center;
}
2. Navigate to Content Management > CSS Override. Add this code to the bottom of your CSS file, then press upload.

3. Be sure to Clear cache in order for the changes to take place.
4. You now have a custom MegaEdit background!

Please note this is a global change and will affect all of your MegaEdit products.