I did some tests and it seems to only save images shared from other LG G5 phone users, or those that use the Advanced Messaging feature that shares larger files, get read receipts and see when someone is replying. I did try reacting a file named ".nomedia" however it will not allow you to save that as a name without putting something in front of the dot. I saved a file as 1.nomedia and it still would save the images. I then turned the Advanced Messaging off and it stopped saving the images in the .temp folder. I deleted the .temp folder and all the images in my messaging app disappeared. This problem only started with I updated to Android 7.0.