You could run a loop, identifying PDF and images, and converting images to PDF with ImageMagick. When you're done, you assemble it all with pdftk.
This is a Bash-only script.
#!/bin/bash
# Convert arguments into list
N=0
for file in $*; do
files[$N]=$file
N=$[ $N + 1 ]
done
# Last element of list is our destination filename
N=$[ $N - 1 ]
LAST=$files[$N]
unset files[$N]
N=$[ $N - 1 ]
# Check all files in the input array, converting image types
T=0
for i in $( seq 0 $N ); do
file=${files[$i]}
case ${file##*.} in
jpg|png|gif|tif)
temp="tmpfile.$T.pdf"
convert $file $temp
tmp[$T]=$temp
uses[$i]=$temp
T=$[ $T + 1 ]
# Or also: tmp=("${tmp[@]}" "$temp")
;;
pdf)
uses[$i]=$file
;;
esac
done
# Now assemble PDF files
pdftk ${uses[@]} cat output $LAST
# Destroy all temporary file names. Disabled because you never know :-)
echo "I would remove ${tmp[@]}"
# rm ${tmp[@]}