creating thumbnails/ screenshots

Hello,
I was looking for a small app ..for taking screenshots like we see on the forum.

I currently use thumbnail.me 3.0 version but the real problem is that it dosn't support some file extensions.( .mkv) It gets stucked or dosn't give us the number of screenshots (it gives blank screen) ...
I want to post but not able to do so due the application error
Can any one suggest any better app ..?

SUU Design Video Thumbnails Maker

thanks NUCKY