Thank you for imagining music!
Everything was drawn directly in Decker. If you do a few things (turn on the toolbars, grab the extra brushes from the included examples folder, switch to color if you want color) Decker is a nice little art program.
If I'm working on a background I'll blob some color around for the important landmarks and refine it when I have time.
If it's a character or object that I'll want to be able to interact with using scripts or modules I'll usually draw it on a blank card, copy and "paste into canvas", and then copy the canvas into the real card it belongs to when it's done.
I had a lots of spare 'doodling/art development' cards over time and would delete them when I was finished. (Whenever I needed to adjust art in a canvas, I'd "copy image" from the canvas and paste the art onto a new blank card for edits.)
Beyond that, I'm still figuring out my workflow, but I hope something in this helps.