A deleted answer was on the right track. A slight modification to your attempt:
shopt -s extglob
MYDIR="./images"
OTHERDIR="./images/junk"
SUFFIXES='@(pdf|eps|jpg|svg)'
mv "$MYDIR/"*.$SUFFIXES "$OTHERDIR/"
Brace expansion is done before variable expansion, but variable expansion is done before pathname expansion. So the braces are still braces when the variable is expanded in your original, but when the variable instead contains pathname elements, they have already been expanded when the pathname expansion gets done.