Stretched or squished screencaps?
 

I am not picking on any one IMFDB member (and if I posted your pic, it was random) and I've seen several IMFDB members doing this.

I am confused as to why some people's screenshots are 'squished' i.e. anamorphic distortion from widescreen to full screen.

Example:
http://www.imfdb.org/images/a/a3/CSI...Test_Image.jpg
This was posed on a page when it should look like this.
http://www.imfdb.org/images/0/0d/CSI...TestImage2.jpg

I know some folks have little control over their screencapturing software, but why is this occurring?

Usually, it's a matter of adjusting aspect ratio and/or screen resolution. I used to get "squished" images when I screencapped from some of my DVDs using VLC Media Player, but I played with settings and finally got them to come out OK. There are some media players which actually allow you to adjust based on the specific size of the frame for the video, so when in doubt, check the resolution or aspect ratio (it's usually on the DVD case).

But some other users on here still seem to have difficulty with this.