With zsh:

autoload zmv # best in ~/.zshrc
zmv -n '(*).(png|jpg|gif)' '$1-mg4sm.$2'

Remove the -n to actually do it.

With zsh:

autoload zmv # best in ~/.zshrc
zmv -n '(*).(png|jpg|gif)' '$1-mg4sm.$2'

Remove the -n to actually do it.

Otherwise, POSIXly, you can always do:

for file in *.png *.jpg *.gif; do
  echo mv -i -- "$file" "${file%.*}-mg4sm.${file##*.}"
done

(remove echo to actually do it).