Since the top two covers were ones I did, I'll comment on this. There are a couple of things that make an image go from peaceful scenery to thriller cover. One is the font. Tall, thin, bold fonts are a signal that the book is a suspense or a thriller. The second is the grunge overlays applied to the image. A little bit of grunge can signal that things are about to get a bit dark. But you don't want too much, because that's generally a signal that the book is really dark, or straight-up horror. Another thing is the vignette to darken up the edges, although that's not really just a suspense/thriller thing, that's just generally a professional designer thing, to help give the cover a more finished look. And there's also the color palette. Color theory is not just for art class. A good designer knows how to use color to help signal genre. For suspense and thrillers you want bold colors and good contrast. With the top three, they all used good color contrast and thrillery colors. The Matthew Rief cover isn't using thrillery colors so much, but the red water is a good signal that things are about to get dark.
Gregg, the font you're using is a middle-of-the-road kind of font. It doesn't really signal any particular genre strongly on its own. It's not bad, but if you want to signal thriller more strongly with your typography, I would suggest looking at Steelfish, which is a free font. I think you can find it on FontSquirrel.com.