Use ffmpeg to extract 15 frames at specific intervals
The basic template is
ffmpeg -i inVideo.mp4
  -vf &quot;select='between(t,30,30.5)+between(t,45,45.5)+between(t,73,73.5)'&quot;
  -s 320x240 -vsync 0 out%d.png

The select filter expression selects all frames in each of the three ranges. I've chosen an endpoint 0.5 seconds greater than the start point under the assumption that the video is 30 frames per second. Adjust accordingly.
Frame sizing is an output option so it has to appear before the output it is meant for.