Note, often you can reduce the file size by reducing the resolution (rather than overall size). For printing, you want 300dpi or 600dpi, but for web use, 96dpi is more than sufficient. Copy the files to a subdirectory, and then reduce the resolution of the copies, should solve it.
You can also save a lot of time by taking screenshots as jpegs in the first place. For example, in VLC go to Tools--Preferences, hit the video tab at the top and adjust the Format in the "Video snapshots" section (pic below).
hope said: I do it the low rent way by dropping pictures into PowerPoint, cropping them and reducing size, and saving each slide as a jpeg.
Assuming you're using Windows, Paint will do that more easily. Crop the image, use the Resize option to shrink itm and "Save As" to save it as a different format.