My GS4 can't send MMS to iPhone users?

I got a new Samsung Galaxy S4 and it can't send pictures to users who don't own a Samsung… It says on my text messages that the picture is sent, BUT my friends keep on telling me they didn't get any pictures:/ Any solutions? And is this happening to anyone else who owns a Samsung Galaxy S4?

Tell your friends to turn on sms and mms. They can receive pics from androids.