I use PowerPoint at work to make infographics. Its quite versatile. Never thought about thumbnails!I might get some criticism and laughed at for this, but I actually use Powerpoint to make my thumbnails. Powerpoint is preinstalled, I have it on ALL my machines and it has all the tools I need, plus some new fonts that I downloaded.
I think Powerpoint is under-rated. Its just a tiny thumbnail. Plus, I save all my old thumbnails on previous slides and just format paste on anything I want to reuse.
Its even got crude photo editing software in-built.