I might be giving a brief talk on what Game Maker is and demonstrating the process of making a simple game in GM. If anyone has a specific sprite or song they'd like for me to feature, give me a buzz and I'll see what I can do. I won't have time to get very far in the development process, so I won't be able to feature a lot of assets, but it is an opportunity to get a bit of free Real Life Publicity for your work. (There's also a chance that I'll be speaking on another topic, so I can't make any guarantees.)

I don't have anything to offer, but surt has a lot of public graphics on OGA like [url=https://opengameart.org/content/monkey-lad-in-magical-planet]Monkey Lad in Magical Planet[/url] or [url=https://opengameart.org/content/blowhard-2-blow-harder]Blowhard 2: Blow Harder[/url]! Enough for a small platformer or top-down game, at least.

It might be effective for you to make a couple of sprites beforehand, then make an extra frame during the demonstration. That way, you can incorporate Game Maker's sprite editor into the talk and maintain a cohesive style for the sprites used.