Images 'Free for commercial use and No attribution required" found on https://pixabay.com (retrieved in 2018)

Authors:
spaceship - 'OpenClipart-Vectors'
https://pixabay.com/users/openclipart-vectors-30363/
https://pixabay.com/vectors/black-hole-sun-fantasy-futurism-1295090/

treasure - 'Clker-Free-Vector-Images'
https://pixabay.com/users/clker-free-vector-images-3736/
https://pixabay.com/vectors/diamonds-treasure-glowing-wealth-35490/

Vector images saved as SVG files (can be opened with open-source softwares such as Inkscape)