After opening the video file with MPC and having it at the desired position (which is, stopped at the frame of the movie scene you want to take the screenshot), go to «File» menu and select «Save Image...». It will result in a .bmp file named, something like, "snapshot20070805170649". To take another screenshot just move to the next scene you want to and do it all again. The .bmp file will be renamed automatically. The screenshots will have the same exact size as the video resolution.

Then just upload the .bmp files to TinyPic.com

You can also covert the image in MSPaint from a .bmp to .jpg. This will lower the size of the file while keeping the quality. Open the file in MSPaint then click File>Save As.. At the bottom where it says File Type change it to jpg.