Try this little trick with 'bash' when you need to resize a large number of pictures. Here we use a for-loop, and variable substitution.
We have created a subdirectory called 800/ under the current directory. All the original files are in the current directory, and we want the reduced images in the 800/ subdirectory.
This is all on one line:
for AA in *.jpg; do echo $AA; convert -size 800x600 -resize 800x600 $AA 800/$AA; done