Preload image and text to the custom product

Open the Custom Products page, edit any available custom product. 

In the custom product editor page, you will find 'PRELOAD TEXT/IMAGES' area like on the following screenshot:

Click 'Add image' to add an image to the custom product, you can, Click Add text to the text to the custom product. The popup which containing some parameters to set the image or text will be showing up once you click these buttons.