Hey! All of the assets have to come from the pack, you can't use any images you made yourself or have sourced from anywhere else. As for your sample, this package has some stars - so together with a solid color background it can be made to look like space.
And I've just noticed some moons in this one 😋 https://kenney.nl/assets/background-elements-redux
Thanks for heads up tho, good to know about rule limitations 👍