File Icon Preview sometimes Movie Poster sometimes movie scene? and 4K playback

Some files i think mainly mp4's I come across have the actual movie poster as the icon and some files have just a random scene like halfway into the movie or something as the icon when viewed as medium icon. Is this something embedded into the file itself or is it a setting. Should I check the 4k in the Lav video player if I am using AMD A10 processor and AMD radeon R9 270x graphics card? I am using cpy back and Enhanced custom presenter. Seems madvr does not like 4k playback with my system. Thanks!


AMD A-10 3.9ghz
AMD Radeon R9 270x 4gb Graphics card
16GB Ram
Samsung SSD
WD 7200rpm 1 TB hard drive for 4k content
Sony XBR900E tv
All Newest standard HDMI Atevon

Yes, if the file contains an embedded cover, then that will be used as thumbnail. Otherwise it will use a frame from within the video (by default at 25% of duration).

You can customize this behavior here:
Codec Tweak Tool > Thumbnails > Options