Hi Godelian,
Take a look at the thumbnail guidelines.
I did feel a “sense of threat, dread or terror” when opening your first thumbnail. (Maybe the image on the left constitutes a ‘grotesque creature’…)
With the second thumbnail, It seems fine, but I think it goes against the “Be engaging” guideline.
I’d try again with that text on a colored/textured background (a screenshot of your island maybe) and/or with the font embellished.