Is it possible to show thumbnails (instead of icon) on video files with Media Player classic without installing extra software like codec pack or Icaros shell extensions?
In that case, you'll need Icaros or something similar. (I asked what version of Windows you are using because Windows 10 has thumbnail generation for MKV and MP4 files.)
for most file types its just a simple matter of changing the icon size to anything bigger than small, but as said above sometimes icarus or similar is needed