With dumper, you can download whole image galleries of the most popular boards.
It's provided with many modules, including 4chan, deviantart, imagebam, mangaeden, yandere, etc.
It's customizable (you can choose how many pages or images it has to download), clever (it detects automatically what modules is required) and extensible (you can add the support for new websites just adding their module).
To install, the only requirement is having a ruby interpreter installed.
$ gem install image-dumper
For more infos, check out its page on GitHub.