This will take a folder full of images, resize them to the target size and stitch them side by side

wallpapers.sh

SCREEN_RES=2560x1440

mkdir result 2>/dev/null || true
for a in *.jpg; do for b in *.jpg; do
  [[ "$a" != "$b" ]] && \
    convert \
      "$a" -resize "${TARGET_RES}^" -gravity center -crop "${TARGET_RES}+0+0" +repage \
      "$b" -resize "${TARGET_RES}^" -gravity center -crop "${TARGET_RES}+0+0" +repage \
      +append "result/${a}-${b}";
done; done