Credit the creator
When you use an image from Commons outside of Wikipedia, it's best to provide a credit line.
You can find the credit line by clicking the "Open in Media Viewer" button beneath the image on Commons.
By pushing the download icon on the bottom left, you'll be given the option of downloading various sizes, but also prompted with "You can attribute the author."
Choose that, and then decide if you want a plain-text credit (for print projects) or an html credit with working links (for the web).
Apply them to wherever you use your image, and you're covered!