It's not more - only place different sprites
1.) On Start & repeat X times = create "Stars" & "Nebula" on randomX and randomY (window height and width)
2.) On created "Stars" & "Nebula" = set frame to random (FrameCount), set random scale (for different sizes), set random angle, set random opacity,...
3.) On DoubleTab = Restart Frame
4.) On TabHold = Make "Screenshot" (there's an great Tutorial how save screen as image)
And the Images are all handmade
But it's very ease to create those Stars and Nebula (i use Corels Paint Shop Pro X5).