Why wordpress images only work with only one domain

I have created many WordPress sites and there is something I was never able to fix.

When you have two domains for the same website (such as www.example.com and www.example.fr) only one shows correctly and the alternative doesn't show it's images.

I guess this is a common problem that might happen to a lot of you. Any idea to help me fix it ?

Thanks!

via Chebli Mohamed