File names with spaces in BASH

I'm trying to write script that'll crop and resize large photos into HD Wallpapers.

#! /bin/bash


for i in `ls *.jpg`
do
    width=`identify -format '%w' $i`
    height=`identify -format '%h' $i`

    if [ `echo "$width/$height > 16/9" | bc -l` ]
    then
        exec `convert $i -resize 1920 -gravity Center -crop '1920x1080+0+0' +repage temp`
    else
        exec `convert $i -resize x1080 -gravity Center -crop 1920x1080+0+0 +repage temp`
    fi

    rm $i
    mv temp $i
done

But it seems that the script has problems with file names having spaces (like Tumble Weed.jpg). How can I fix this?