Hello. You are great and I don't want to use an external issue tracker without the quality UI that you guys provide. I'm allergic to bad design and I love you for being my medicine. I'm building an app with UI as well and my client needs to report issues with screenshots most of the time to present what he perceives as the problem. There is no way to attach files on issues at the moment and that is making Github Issues useless for things that include UI on web, mobile and desktop!
I googled and found this extremely complicated thing: http://feeding.cloud.geek.nz/2012/06/attaching-files-to-github-issues.html and I wanted to let you see it and hopefully you will get worried about what people do in order to complete a task that isn't weird. I understand your well funded obsession to keep a clean core of features, but I think this is something you should honestly fix.
Please don't make me use anything else. As a coder I know putting attachments is pretty simple. You can list them like in emails, they don't even have to be show inline to be useful.
Please let me know what you think!