It is important that your media file complies with the KakaoTalk's criteria, which is defined in the Media Messages section of our KakaoTalk developer documentation:
Media included in the Media message will be rejected by the channel if:
Double check your image and make sure it's in the right format before resending your FriendTalk message. Here are few examples you can use:
The Sinch Developer Documentation site contains comprehensive documentation that illustrates how Sinch supports KakaoTalk through the Conversation API.