I work on Aircrafts ( lot of panel lines, rivets, labels, markings etc ... And background image as reference feature is more than required. This basic feature present in Blender and everywhere else seems to lack in Substance, am I right ?
I understand Substancer Painter works differently than a casual 3D modeling software, still I need this feature, like in any CAD, to add a " transparent " plane, with an image texture glued to it, with opacity settings to then paint on the mesh folowing it as a template.
As a workaround, you could project the guide image onto the model surface base colour, in a new layer, and use that as a guide. Lower the opacity whilst painting, then just turn off that layer or discard it when done.