Nice work; I love the presentation. Thanks for contributing!
One refinement you could make is adding transparency information. With a drawing tool active, toggle the "View -> Transparency Mask" menu item. This will show the transparent part of an image in a mid-gray:
Normally, the "white" color in the palette is transparent. In Transparency Mask mode, that color allows you to draw in "opaque white":
To see the effect of this tweak on contraptions, you'll need to make them transparent by selecting them with the Widget tool and then choosing "Widgets -> Show Transparent" in the menu. With both of these changes, it's possible to overlap contraptions and have part of the lower contraption show through. "Widgets -> Show Inverted" is a bit similar, but it toggles pixels that overlap, which can produce some interesting effects:
(Everything about the transparency of a contraption background also applies to the image on Canvas widgets, too.)
Just something to think about!