From my sister's phone many photos has been sent to others on WhatsApp

So Today suddenly I got a image from her on WhatsApp..i thought it was a random picture but i couldn't download so I asked what what was it...and she told i didn't send you any picture and worst part it's not even showing on her phone that any picture has been sent. And then she called and told me that many images has been sent to 4 -5 people without her knowledge. Please if anyone has any solution or advice please tell