Something it easier to do something as a batch. Normally I'd do this kind of stuff in bash... but today I was on a Windows machine so...
First I had a bunch of files with double extensions. Why do it by hand when a one liner will fix it right up:
for %f in (*.jpg.jpg) do ren %f %~nf
Next I re-sized them all using convert with this one liner. The '^>' is an escaped greater than telling convert to only fiddle with the larger images.
for %f in (*.jpg) do convert.exe -resize 400x400^> %f resized/%f