As a general rule of thumb, do not use stock or clipart images.

Most licenses for such images are non-transferable (meaning you can’t just give the license to the contest holder), and don’t let you use the image in a design which is to be trademarked (such as a logo).

If you really want to use stock images or clipart in your designs, please read when you can use these images.

Keep in mind that contest holders aren’t going to be happy if they are legally pursued over copyright issues arising from your work — and you’ll be liable for any costs and damages they incur!