Once it’s a vector, yes, of course removing the outline inverts it. That’s how vector fills work in LightBurn - Each edge encountered toggles the fill. Read here: How do I invert (negate) a vector engraving?
For the image, I can’t see enough of your screen to tell what’s going on.
An image WILL invert when previewed, but if you invert it, the preview might also invert automatically, which you can easily turn back off again.