ffmpeg and jpegs
The documentation for ffmpeg on manipulating single jpeg frames is a bit obscure. As of the time of this writing the FAQ on using jpegs is incorrect. It suggests using the -img jpeg switch which will not work. There are a couple things that are important. 1) the -img switch won't work. Don't use it. 2) it expects "%d" in the output file format. Maybe in the input if you are assembling jpegs as well. I haven't tested that.
ffmpeg -i file.avi -ss 00:00:00 -vframes 1 -s 150x100 test%d.jpeg