Hi Alex,
If it turns out that your images have a license which does not allow re-use, then you might need to swap them out for similar ones from a site that does allow re-use (perhaps from one of the links below).
This is the header from our 'Space Shooter' demo for your reference:-
Note: that Chris has not specified a license term for his code which means it will default to being Copyright Chris Hopkin and no-one will be able to use it in their own work without his explicit permission.
I would encourage you to be a bit more liberal than him and specify that your work is covered by the MIT license, then others could use your game demo as the basis for their own work and it will help build your reputation in the industry as your source code spreads far and wide ;)
////////////////////////////////////////////////////////////////////////////////
// Space Shooter v1.0
// 07/10/2014 - Chris Hopkin
//
//
// Explosion Sound Effect by dklon (CC BY 3.0 license)
//
// Fighter Graphics by MillionthVector (CC BY 3.0 license)
//
// Music by Jan125 (CC BY 3.0 license)
//
// Alien Ship Graphics by surt (CC BY-SA 3.0 license)
//
// Background Graphic by freedohm (CC0 1.0 Universal license)
//
// Bullet Sound Effect by Luke.RUSTLTD (CC0 1.0 Universal license)
//