Well Patrick,
it looks like your image is a Mesh (maybe through Import Image As Planes) and that Mesh is in Sculpt Mode (you do not want to Sculpt, you want to draw and the Sculpt/Draw Brush has nothing to do with drawing…)
If you want to use Grease Pencil, Select the Grease Pencil Object (and not something else!) and switch to Draw Mode.
You can also use the Annotate Tool (Toolbar > Annotate), no GP Object needed for that.
