Standard Definition TV screens do not have square pixels like on a computer. For 4x3 standard definition they are slightly taller than the are wide. However, with 16x9 widescreen, the image is compressed horizontally and then stretched back to normal by the DVD player or TV.
Basically, your media player is taking a screenshot of EXACTLY what was on the DVD at that frame. Some players will automatically stretch the image to match on screenshot, but MPC doesn't, so you will have to do it manually in a program like Photoshop to 848 x 480.
Fuck. Well, I don't have photoshop, so that kills WMP Classic for me in so far as taking screenshots is concerned. PowerDVD was great like that, you could set what size you wanted the screenshots easily.
IrfanView is good for things like this. I suggest setting it to resample rather than just resize, though, otherwise it'll look rather blocky.
どうして?お前が夜に自身お触れるから。 Long ago in a distant land, I, Aku, the shape-shifting Master of Darkness, unleashed an unspeakable evil,
but a foolish samurai warrior wielding a magic sword stepped forth to oppose me. Before the final blow
was struck, I tore open a portal in time and flung him into the future, where my evil is law! Now, the fool
seeks to return to the past, and undo the future that is Aku...
-Aku, Master of Masters, Deliverer of Darkness, Shogun of Sorrow
Also, Paint.NET is a good free image editting program.
"preemptive killing of cops might not be such a bad idea from a personal saftey[sic] standpoint..." --Keevan Colton
"There's a word for bias you can't see: Yours." -- William Saletan
Another player that's both very cooperative in playing various file formats and takes screencaps (only 320x240 I believe) is GOM Player. I actually prefer it to VLC player because it has a playback priority setting which determines how much the program can tax your CPU. A great way to improve file playback if your machine is old or having problems.