ITT BASH

#!/bin/sh

jpg="$1"
shift

file=$(mktemp)

rar a "$jpg.rar" "$@"
cat "$jpg" "$jpg.rar" > "$file"
mv "$file" "$jpg"
rm "$jpg.rar"

>>1
Doesn't work on the image boards anymore. Try lhz, though even though WinRAR handles jpg + lhz just fine, the Unix version of lha won't. I keep wanting to make a patch to lha to keep searching the file for lhz headers instead of giving up if the header doesn't start at the first byte, but it's not like anyone other than me uses jpg + lhz on 4chon.