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Do you still have the text message with photo ? If so, then you have to download that photo to your phone first. To do that, tap and hold down the photo on the text message and it will give you an option to save the photo. Then save it.
Or open the phone, tap the Options button or menu button and you will see the option to save the photo to phone. Once you did that, you will find the photo in gallery. Then you can send it via email.
If you still have trouble saving it, then let me know the exact model number and make of your phone and I will send you correct instructions to do that.