Add Pomf.se to accepted image hosting services

I've seen many people use Pomf. Besides, it's the only one (I believe, I'm not completely sure though) that doesn't compress/resize my screenshots (3840x2160) - in contrast with, for example, [Imgur](http://help.imgur.com/hc/en-us/articles/201476437-What-is-the-maximum-file-size-for-uploads-), which doesn't allow anonymous users to upload high resolution pictures.