Throwing away nearly all the speed gain in the process

If you could post the literal contents of one of those files, that'd be good. I built it to work with your test data. If it's actually any different, then I really need to see what it is. The built-in substitution operator is hundreds of times faster than calling an external utility. At the very least the builtins instead of basename and basedir ought to be better. The rest of my code may have depended on certain assumptions about your data.

Are these folders full of irrelevant files? If so, the for FILE in "$MOVIEPATH"/* loop will waste a lot of time. Come to think of it, since we're only interested in .jpg, you can make it for FILE in "$MOVIEPATH"/*.jpg

---------- Post updated at 09:29 AM ---------- Previous update was at 09:20 AM ----------

Here's a version which works without trawling every file in the folder. You can replace the long if-else chain with two for's. It also helps make the lists longer without making your code longer (though testing for too many things will slow you down in any case).

Code:

for FILE in "folder.jpg" "${MOVIENAME}.jpg"
do
        [ ! -e "$MOVIEPATH/$FILE" ] && continue
        MOVIEPOSTER="$MOVIEPATH/$FILE"
        break
done

for FILE in "about.jpg" "0001.jpg" "${MOVIENAME}_sheet.jpg"
do
        [ ! -e "$MOVIEPATH/$FILE" ] && continue
        MOVIESHEET="$MOVIEPATH/$FILE"
        break
done

---------- Post updated at 09:35 AM ---------- Previous update was at 09:29 AM ----------

And you can strip out this completely:

Code:

        if ! [ "$MOVIEPATH/$MOVIEFILE" = "$LINE" ]
        then
                echo "Error processing line" >&2
                continue
        fi

I just put it there in case your input data was radically different from what I assumed it was.