emailing a photo from my pc to an at&t cell phone

i am trying to email a friend a picture from my lap top via windows live mail to his phone. its an at&t cell. iv tried the address where u insert the receivers phone number then add .txt.att.net but it will not send at all....it says its not a valid email address. iv even tried putting a www. at the beginning. i remember years ago there were a couple of ways to email an at&t cell from ur computer, but to be honest, i just cant remember....please help???
thanks

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Most carriers have email gateways to facilitate the transfer of messages between telephones and email accounts. You can find the most common ones here: List of SMS gateways - Wikipedia, the free encyclopedia
Note that the gateway for SMS only allows text to be sent and has the one hundred sixty character limit per message. The MMS gateway allows the sending of audio, video, pictures and text. There may be a size limit for multimedia files sent via MMS.

thank you very much!!!!