You can add yours for free - just PR it into this git repo, which is also the source for this site! You'll find the ad requirements in that repo's CONTRIBUTING.md. If you'd like to use these ads yourself, use the following HTML snippet:
<iframe src="https://ads.heyquark.com/ad" width=250 height=190 sandbox="allow-popups allow-popups-to-escape-sandbox"></iframe>
You should stick to sizes between 250x190 and 150x150. You could go a little larger if you really wanted, just keep in mind the ads may not be designed for that. Don't go below 150x150.
Here's a few random ads:
250x190 - the usual size
150x150 - the minimum for heyquark ads