I found some cool Bitcoin pics on https://pixabay.com/. Those pictures are free for commercial use. There is no attribution required (CC0 Public Domain).
Here you can download the pictures in high quality:
https://pixabay.com/en/bitcoin-btc-cryptocurrency-1813503/
https://pixabay.com/en/bitcoin-btc-cryptocurrency-1813505/
https://pixabay.com/en/bitcoin-economy-currency-finance-1995334/