John,
I don't use TurboCad, but … it seems like you have a 'layer order' problem
The layer containing the inserted picture obviously needs to be at the back/bottom of the stack … and if it is to be really useful, it will also be behind/below the grid.
.
Presumably; if there is a tutorial demonstrating this technique, there *must* be tools available to arrange the order of your layers, and/or to insert the picture on a special layer that really is the background. [Note: that special one, sometimes called canvas, may only be accessible when creating the file, rather than when inserting an image into an existing file]
MichaelG.
.
Edit: For example, here is the opening screen from an iPad App that has that option:
Edited By Michael Gilligan on 18/06/2018 06:59:46