Some tips for thumbnails:
1. It needs to be a closeup. Back in the day you could make a thumbnail with tons going on and make it all intricate. While that still works, mobile viewing is getting more and more popular and with smaller screens, closeups are important.
2. It needs to relevant. Your thumbnail should be relevant to the video. Although just randomly putting a picture of a celebrity could get more views, it will thumbs down. Plus YouTube can take your channel down for misleading thumbnails. Make sure your thumbnail relates to the video somehow.
3. Pay attention to dead zones. Places like the lower right corner where the time code is displayed and the edges which get cut off on most devices are not a good place of images or text. Try to avoid placing things in those areas.
If you are looking for a good thumbnail program that is free and easy to use, I recommend picmonkey.com it only takes like ten minutes to figure out and you can make some cool stuff. There are definitely more professional software out there, but they cost money and are harder to use. If you have time and money I recommend Photoshop it is what most big YouTubers use. However, you won't be able to learn Photoshop in ten minutes.
And the dimensions of a thumbnail should be 1280X720 for best results. Keep that in mind.