This is the "reader"

#!/bin/bash

cd /home/cornizzolo/webcam
rm -rf fifo work
mkfifo fifo
chown cornizzolo fifo
mkdir work
while true
do
        read file <fifo
        convert -size "640x480" $file work/$file
        curl -F uploadedfile=@work/$file http://url/webcamup.php
done


And this is the "writer" (there can be 4 at the same moment):

#!/bin/bash

cd /home/cornizzolo/webcam
read file1
file2=`date -u +%Y%m%d-%H%M%S`-`basename "$file1"`
cat >"$file2"
echo "$file2" >/home/cornizzolo/webcam/fifo