Honestly, I think they need some improvement.
They possess a style very similar to the generic Minecraft thumbnail.
(Minecraft background and basic coloured text)
You want to avoid this as it doesn't help your videos stand out and draw viewers in.
Here are a few thumbnails that personally stand out for me and reasons as to why:
1.
http://gyazo.com/19d7b5e74d70526dd60db6ab80db859f
It's text focused and the art presents a fun cartoon feel.
Personally, If I see that a thumbnail has a lot of thought put into it and provides a professional feel, Im going to expect the same from the video itself.
2.
http://gyazo.com/3eff9203d33c0b0ba561b71a5534549d or
http://gyazo.com/bbe7f9074bb9484b432c71b825e7021d
The top videos related to minecraft, contain the youtubers character skin in 3D.
Unlike the last one in which you have to be good at art, you just need to find out how to import your Minecraft skin onto a 3D Minecraft model.
From there it depends on you, whether you want to add text or not (sometimes it looks good without it).
As of the moment you just need to experiment more and refine them
Ensure that the thumbnails all look consistent and provide a similar feel.
That way your subscribers will know straight away who the video belongs to just by looking at the thumbnail.
Good Luck!