I can't give you a definitive answer, but I can think of several possibilities:
- Your account is not provisioned for MMS by your carrier (assuming you are trying to send them via messaging and not email).
- If you are trying to send them by email your email provider has a size limit on messages that is lower than the size of the pictures you are trying to send.
- You haven't turned on MMS messaging in Settings/Messages
- You don't have an Internet connection when trying to send pictures
- You have turned off Cellular Data and don't have a WiFi connection when trying to send pictures
You need to change your APN settings this should be the full "how to" for the 4S- have a great day! If anything just watch the You Tube video and that will explain a "work around".
To clarify for anyone who is still having any problems doing this.
1. Unlock your phone with the carrier ( I know the process will work if it is jail broken but I am assuming you dont want to void the warranty on your phone) NOTE: even if you purchased the phone outright at Wal-Mart it is still locked to ATT so you will have to unlock the phone.
1a Here is the link to unlock a ATT iphone. Simply follow the instructions https://www.att.com/deviceunlock/client/en_US/
2. Get a T- mobile SIM. Even if you don't want the service, if you feign interest, they will send you one for free or 99cents. If you need it now you can get one at a T-mobile store too.
3. Once you have both your Straight talk SIM and your T-mobile SIM follow these instructions:http://www.youtube.com/watch?v=mFFf5uqk18M
4. If you have any remaining questions look at all the responses posted previously on this thread or check out the Howard Forums Wiki: http://wiki.howardforums.com/index.php/Straight_Talk_iPhone
Everything should work flawlessly at this point. I hope this makes doing this very easy for everyone in the future. God Bless -J