I am not saying not to trust the word of the CEO, but this exact use case of using cloudflare as a image hosting comes up a lot on HN.
The word on the street is that they will start throttling and contacting you once you hit several hundred TB per month. [1][2][3][4][5][6]
Of course this is still extremely generous and the upgrade plans are usually still several orders of magnitude cheaper than any cloud provider per gb. But don't build a business or hobby project around cf providing unlimited free bandwidth forever.