According to user reports, the problem stems from Samsung Messages, the default texting app on Galaxy devices, which (for reasons that haven’t been determined), is erroneously sending pictures stored on the devices to random contacts via SMS.
Samsung back in the news again for all the wrong reasons. This time it seems their top of the line phones have begun texting random photos from your phone to random contacts.
The bug appears to be linked to Samsung Messages app, the default texting app on Galaxy devices.
The fix, for the time being, appears to be “use another messaging app” and revoke Samsung Messages’ ability to send texts.