Maybe someday we can attach images/docs to issues... I mean, why would you ever want to attach an image to a site rendering issue when you can try and describe it at length and still fail? Sigh.
Multiple examples in this PR: https://github.com/clayallsopp/formotion/pull/24
However, for the specific case of misrendered HTML skitch is def the best option, even if your software supports attachments. So few clicks, totally worth it.