I use Sony Vegas for my thumbnails.
I could never figure out how to do outlined text in GIMP 2 so I did it the way I already knew in Vegas.
Add some images, typically relating to the video, some text if necessary and screenshot the result in Vegas.
Instant 1080p perfect aspect ratio thumbnail.
Here's an example of one of my thumbnails; The video was for when a Putin ally said "Vote Trump or face nuclear war".
Usually don't do anything more complex than this, it is only the thumbnail after all.