Well this is a little silly Google Docs does not allows an image to be in the background, you can have it inline or it can break text, but not in the background. The hacky solution is to create a drawing and this does support layers. Doesn’t work for multipage, but not bad for titles and things where you want a background.
Hat tip to https://webapps.stackexchange.com/questions/51613/how-do-i-create-background-image-in-google-docs
