There are 3 options:
1. According to the wiki page, you can download the image they provide but there’s no kernel support for Mali400MP2 GPU (who knows if it’s fixed by now). No Mali mean no GUI. The link to image is http://download.opensuse.org/ports/armv7hl/tumbleweed/images/.
2. Download the image from http://www.lemaker.org. The GUI used is XFCE.
3. Do it the hard way, build it yourself. I would like to install MATE. I know, I could use the lemaker image.
I followed the page HowTo Build Banana Pi Image.
This post has 2 sections. The first is how to create the SD card and the next one is how to install MATE.