If you mean that you want to add a default image that will appear in every scene by displaying that image in a window via window editor, just remove the standard window images that are there--there are 2 -- "Image" and "Image Inactive". To remove the images that are there by default, hit the three little dots and select "remove assigned file", then where you see the blank white box and the three little dots, hit the dots and connect to your image file.
Then you have to code your game to display that window. I'm not sure if this is what you're asking, but I hope it helps.