It's common to want to demo your app on GitHub to visitors, either through pictures or a short video snippet. Here I take you through 2 ways of doing so.

In your README

Most of the time, you'll want to use them in your project's README. In that case, you can simply drag and drop either your picture or video into the text window on Github. Once you do, you'll see it uploading and then a link in the text window, shown below. Click submit and you have your uploaded file:

In your Wiki

However, adding media to your repo's Wiki requires another process, as you cannot (at this time of writing) simply drag and drop files into it. Instead, navigate to the "Issues" tab. Once there, click on "New Issue", like so:

Then, simply do as before and drag and drop your media file into the text window. After uploading you'll have the link to your uploaded file wrapped inside an img tag, like so:

Simply copy and paste that text into your Wiki text window, and you're done!