How do I prevent the outline from dipping into an image that has gaps inside it?

[rich2005]

Remember - Gimp is a graphics editor. It only does what you tell it to do.  You are getting good practice with your images.

One thing with the snowman. A very common problem for beginners. There are some white-ish 'boundary pixels' left over from the background. The usual way is shrink the selection by a pixel, invert and cut to get rid of them. (then invert selection back again) In this case it leaves the border a bit 'jaggy' . Another way is make the selection, set the brush to a small size (3 pix) and stroke the selection with the eraser tool. All sorts of ways in Gimp, you will find your own favourite methods with time.

Stroking the selection with grey (3 pix) now gives this :  https://i.imgur.com/CE5m76S.jpg


The gimp xcf.gz (gimp compressed format - opens as any other gimp file)  attached with the selection still active. A gimp xcf can retain selections / masks / layers ...   all sorts of things but not undo history. Make sure you keep backups as you go along.