We don't have roadmaps or a prioritized backlog, so we weren't exactly "waiting" to implement this. Someone got fed up with the image workflow, figured out a solution, grabbed some other people to help, and shipped it.
Now I know github doesn't have a typical roadmap or backlog but I guess what I was looking for was how someone decides what feature they are "fed up" with not having.
We use GitHub to build GitHub, so typically a feature comes from our own usage pain points.
I imagine the conversation internally went something like, "Man, attaching an image to an issue is really annoying. Let's make it not suck." An issue and pull request followed, internally shipped it to test it, and today rolled it out to you all.
That's literally how 99.99% of features happen on GitHub. :)
I was working on a project where, when viewing an issue, you could use the file picker to choose a file that was then committed on the project's 'gh-attachments' branch, and then linked via markdown to the raw URL.
Someone came up with a design. Someone else implemented the backend. We iterated on it for a bit and shipped it internally. Then we figured out the CDN situation, setup logging and metrics, and shipped it genpop today.