Most of the vectors in images.svg/buttons.svg are custom drawn by hrydgard and some by NABN00B.

Some are derived from public domain SVGs from the following sites:

https://www.svgrepo.com
https://uxwing.com

"Flag of Europe" -- Copyright © Council of Europe
See the Conditions of use here:
https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:42012Y0908(01)&qid=1530786690534&from=EN
