Why can't I send pictures to non-iphones?

I have the iphone 5s and ios 7.
I tried sending pictures to my friends who don't have iphones but I just get an error message. But when I send pictures to iphones using imessage, I'm able to send pictures just fine.
Can anyone tell me how to fix this problem?

Maybe you don't have MMS enabled? Check your messaging settings. Also, make sure you have your cell service on, and make sure that airplane mode isn't on. MMS uses your cell provider's service to send pictures and it may not always work, while iMessage uses the internet to send pictures so it would work with data or Wi-Fi.

Iphone only allows data transfer between iPhones to iPhones and you can't do sharing to other phones until you jailbreak it, and by looking at your question I think you are an amateur iphone user, so for sending photos you can use what's app or email, because iMessage only works iphone to iphone and Bluetooth works the same way in iphone and if you try to send message anyway, it will be charged as mms