Why do my iMessages get sent with someone else's information?
Is the default number or mail address associated to iMessage stored in your contact card? If not this is the reason.

You can change the default sender by coning to Settings, iMessage and setting the field Start new conversation from:.

This phone number or email address will be the one used for sending iMessages. Hence, if this number is not associated to your contact, iMessage will not show your picture.

If all the available options belong to someone else, then check that the contact card for you is assigned correctly.