Why can't I save some images on my free account?

Your free account allows you to use most features, so you can get a feel for Stencil. However, on the free version you can only Save images that aren't using locked photos or icons. Locked photos and icons are for Pro & Unlimited users only. The first 24 photo and icon search results are unlocked and totally free for you to use. 

The rest are only for Pro & Unlimited users and are indicated by the small red lock icon when hovering the thumbnail, as seen here:

You can still load locked photos or icons into the canvas and create images with them, you just won't be able to Save until you upgrade to a Pro or Unlimited account. If you have a locked photo or icon in your canvas, it will be indicated by this icon on the bottom right corner of the canvas: