Copypasta related on how to take screencaps in MPC:
1. Open MPC
2. Open options
3. Go to Output
4. Select Haali Video Renderer
5. Click OK
6. Open a video
7. Go to File -> Save Image
8. Save image where you want your snapshots to go, make sure to save in JPG
After this, press F5 whenever you want to take a screenshot and it automatically saves a JPG snapshot to whatever folder you saved the first image in.
If you experience slowdown with Haali Video Renderer, VMR7 (windowed) is the fastest renderer that allows you to take screenshots.