Hi Emilio,
I'm not sure if I've replicate this correctly or not as I don't have an existing account in your site, I've tried to register using Facebook and Twitter using username (Fadhli and Warith) respectively.
The avatar and cover/background image was imported correctly for Facebook registration
but for twitter seems like it does not get the cover image, I've logged this issue in our issue tracker and our developers will take an action for this.
If the user is already registered before,user need to link the Facebook/Twitter account in order to fetch the avatar and cover image