If you like xargs, but want jobs to execute in parallel, you may enjoy GNU parallel.
Your trivial example:
find images -name "*.jpg" | parallel convert -geometry 200x {} {.}_thumb.jpg
Your trivial example:
find images -name "*.jpg" | parallel convert -geometry 200x {} {.}_thumb.jpg
GNU parallel makes sure output from the commands is the same output as
you would get had you run the commands sequentially. This makes it
possible to use output from GNU parallel as input for other programs.