When I searched earlier for a time for the release of the BG images, the Fox59 site they were released around 7PM on the 15th. I think The phone was recovered, which lead to the video and audio, and then LE released the images on the 15th.
MOO is they recovered the SD card from the phone, which revealed the video. Either the phone was found on Libby, or somewhere along the path taken from the SE end of the bridge, to where they were found. They were found at noon on the 14th, so between then and the release of the images on the 15th, the SD card had to have been recovered, and the video and audio scrutinized by the FBI and ISP.
At midnight on the 13th, LE suspended the search, and said they believed the girls were not in danger.
Early on, we all theorized as to where the images came from. Phone? Trail camera (that was my first thought)?
I think there's a possibility Libby used the selfie cam on her phone, which explains some of the image quality issues.
JMO