I do not know if this is the best way, but it worked cool here:
Youwillneedtochangethefacebookframeworkclassdirectly,ashereIinstalledtheSDKviacocoapods
Icanaccessthefolderscontainingtheclasses:
Then just open the class FBSDKButton
that is inside Pods > FBSDKCoreKit > arc
search for the method that starts with _configureWithIcon:(FBSDKIcon *)icon
and add the following code in the same place as below:
...
BOOL forceSizeToFit = CGRectIsEmpty(self.bounds);
CGFloat scale = [UIScreen mainScreen].scale;
UIImage *backgroundImage;
// Crie um novo UIImage com a sua imagem
UIImage *custom = [UIImage imageNamed:@"backCustomLike"];
backgroundImage = [self _backgroundImageWithColor:backgroundColor cornerRadius:3.0 scale:scale];
// Coloque a sua nova imagem aqui
[self setBackgroundImage:custom forState:UIControlStateNormal];
backgroundImage = [self _backgroundImageWithColor:highlightedColor cornerRadius:3.0 scale:scale];
[self setBackgroundImage:backgroundImage forState:UIControlStateHighlighted];
backgroundImage = [self _backgroundImageWithColor:[self defaultDisabledColor] cornerRadius:3.0 scale:scale];
[self setBackgroundImage:backgroundImage forState:UIControlStateDisabled];
...