If you try to capture a still image from a movie using the "Print Screen" key (or Alt-PrtScr to capture a window), it usually fails in Windows Media Player (WMP), RealPlayer, etc., because of video hardware acceleration.  To get it to work, in WMP, go to Tools --> Options --> Performance, and turn off "Use Overlays".

It is better to zoom the video to the desired magnification in WMP first (because WMP uses smooth interpolation algorithms), and do the screen capture.  If one does screen capture bit for bit at 100%, and later try to blow up an image, the bit fuzz will show (unless perhaps JPEG did some smoothing...).

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Powerpoint often produces larger files than seem necessary.  Some ways to reduce powerpoint file sizes is to clear the clipboard, or to do a "save as".  Powerpoint often doesn't compress images effectively.  A simple way to reduce powerpoint file size without losing any information is to simply save each image as PNG (which does lossless compression) and then reinsert the image from the PNG.

JPEG can compress more than PNG, and can be quite good for photos (with lots of continuous color variations).  However, JPEG compression is never lossless (even at "100%"), and can cause "Gibbs' Phenomena" (ghosts or echos) near sharp variations like lines and text.  For simpler line plots (or with few shaded colors) PNG will be better.

("jpeg rot" occurs with repeated edits and saves of a jpeg file, which leads to some deterioration of image quality.  Apparently it saturates after ~100 files saves.)

Microsoft Digital Image (Starter Edition) appears to be better than the Paint program, making it a little easier to crop images, and saving a thumbnail in jpegs.