To resize all images in the current folder using ImageMagick from bash shell:
for i in *.jpg; do convert -resize 800x600 $i ${i%%.jpg}_800.jpg; done;
So if you had
a.jpg
b.jpg
you get
a.jpg
a_800.jpg
b.jpg
b_800.jpg
where the *_800.jpg images are resized to 800×600.
Enjoy!