What about using yuvfps ? If you know the original frame rate you can simply discard all frames between one and another by converting to a lesser framerate. Say, for example, your data source is 25fps and you want to keep one of each 10 frames. You would then use a 25:10 framerate in yuvfps to convert the stream to that framerate.
lav2yuv <video file> | yuvfps -r 25:10 | .... whatever
The only drawback that I see is that the resulting stream will have a frame rate of 2.5 frames per second. If you only want to generate an image file that's not a problem. However if you want to generate another video stream you can correct the framerate again with yuvfps by using the -c modifier that means don't touch the stream, only change the frame rate in the header.
lav2yuv <video file> | yuvfps -r 25:10 | yuvfps -r 25:1 -c | .... whatever
If I'm correct this approach should be faster because yuvfps simply either discards or copies frames at downscaling/upscaling and you save the time and space of conversion of the YUV stream to another format for frames that are going to be discarded.
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